Update 10.40pm: Swansea made Andre Ayew their first signing of the January transfer window as the Ghana striker returned to the Liberty Stadium in a club-record deal.
Ayew left Swansea for West Ham in the summer of 2016 for £20.5m.
The 28-year-old returns for a record fee, understood to be £18m and possibly rising to £20m.
The move reunites him with his brother Jordan as the pair were previously together at Marseille.
Tottenham completed the signing of winger Lucas Moura from Paris St Germain on a contract until 2023.
Moura moves to Spurs for an undisclosed fee, reported to be £25m, having fallen behind Neymar and Kylian Mbappe in the PSG pecking order.
Update 8.14pm: Sunderland midfielder Didier Ndong has joined Watford on loan with goalkeeper Pontus Dahlberg also moving to Vicarage Road.
Ndong, a 23-year-old Gabon international, will remain with Javi Gracia's side until the end of the season, when the Premier League club will have an option to make the move permanent.
Goalkeeper Dahlberg has signed a five-and-a-half-year permanent deal from Goteborg and will return to the Swedish side on loan for the rest of the season.
Watford also loaned Costel Pantilimon to Nottingham Forest after his loan spell at Deportivo was cancelled.
Update 6.07pm: Manchester City have withdrawn their interest in Leicester winger Riyad Mahrez, Press Association Sport understands.
Update 5.34pm: West Ham defender Reece Oxford will return to Bundesliga outfit Borussia Monchengladbach on loan for the rest of the season.
The 19-year-old, who can also play in midfield, made four appearances for Gladbach in the first half of the campaign before being recalled by the Hammers at the back end of last month.
He featured in three matches for his parent club but it was confirmed that he would go back to Borussia-Park for the remainder of the campaign.
Borussia have agreed a new loan deal for @Reeceoxford_ with @WestHamUtd 🤝
Update 4.53pm: Chelsea have signed Olivier Giroud from Arsenal, with Michy Batshuayi leaving Stamford Bridge to join Borussia Dortmund on loan.
The Blues announced the arrival of France striker Giroud on an 18-month-contract and Batshuayi's departure until the end of the season.
Giroud said on the Chelsea website: "Chelsea is one of the biggest teams in the Premier League. They have won maybe the most trophies in the last six or seven years. It's a massive club and I am proud to sign for Chelsea, and I'm looking forward to playing."
Giroud had become surplus to requirements at Arsenal following the arrival of Pierre Emerick Aubameyang.
Update 4.48pm: Here's a club-by-club guide to every transfer so far in January 2018.
Update 4.18pm: Newcastle are understood to have agreed a loan deal for Leicester striker Islam Slimani.
The Algeria forward joined the Foxes from Sporting Lisbon in the summer, and has scored five goals. However, it has been reported Slimani faces at least two weeks out after picking up a thigh injury in training.
Aleksandar Mitrovic is set to head to Anderlecht, also on a loan deal until the end of the season.
Davy Klaasen's proposed move from Everton to Napoli is off after talks between the two sides broke down.
Brighton have had a second offer to sign Wigan striker Nick Powell rejected.
Bournemouth have had a £5.5million bid for Brentford defender Chris Mepham.
Update 2.57pm: Sandro Ramirez has left Everton for Sevilla on loan.
Some other deals that could be completed to day include a loan move for Leicester striker Islam Slimani to Newcastle.
While Swansea are hopeful of re-signing Andre Ayew from West Ham for £18million.
Update 2.02pm: Leeds have signed forward Tyler Roberts from West Bromwich Albion on a four-and-a-half-year deal.
The 19-year-old has made the move from the Hawthorns to Elland Road for an undisclosed fee and could make his debut in Saturday's Championship contest with Cardiff.
Roberts made just one appearance for the Baggies in the Premier League, coming on as a substitute in a 1-1 draw with Liverpool in May 2016.
Firstly would like to start with saying a massive thankyou to everybody at @WBA for helping me grow not only as a player but as a person. Delighted to announce my move to @LUFC and ready to start a new chapter of my life and career 👊🏽⚽️ pic.twitter.com/iMRpeanVhp— Tyler Roberts (@official_tyro) January 31, 2018
Update 1.05pm: In Scotland, Celtic are continuing their hunt for a goalkeeper after having a second bid for Motherwell's Trevor Carson rejected.
It is understood Motherwell turned down a £375,000 offer for the 29-year-old, who they value at double that amount.
Celtic manager Brendan Rodgers confirmed on Tuesday night that Scott Bain was among a number of goalkeepers under consideration after Craig Gordon was ruled out for 12 weeks with a knee injury.
But the Scotland squad player recently joined Hibernian on loan from Dundee, who earlier rejected Celtic's bid for defender Jack Hendry. Hibs are also reportedly interested in re-signing Scott Allan, who is on loan to Dundee from Celtic, which could spark a complicated three-way negotiation.
The Scottish champions have also reportedly made contact with Newcastle over Freddie Woodman, but Aberdeen are confident of completing a loan deal to sign the England Under-20 keeper, who had a spell with Kilmarnock last season.
Hibs have completed one deal with Florian Kamberi signed on loan from Grasshopper Zurich after training with the club for two weeks. Irish striker Anthony Stokes' departure is expected to be confirmed after a disciplinary issue.
Rangers are continuing to resist the temptation to sell Alfredo Morelos after receiving serious interest from a Chinese club, while they have signed Scotland Under-19 winger Glenn Middleton from Norwich for their academy.
Update 12.41pm: Mesut Ozil has signed a contract to ensure his future with Arsenal until the summer of 2021, according to a BBC report.
The German will become the club's highest-ever paid player, on £350,000 per week before tax.
Ozil's contract was previously due to expire this summer.
Update 12.31pm: Tottenham are expected to confirm the signing of Lucas Moura from Paris St Germain.
The Brazilian winger arrived at Spurs' Enfield training base on Tuesday morning ahead of a medical and completing a £25million switch.
Stoke have been closing in on a deal for midfielder Badou Ndiaye from Galatasaray, while Arsenal look set to allow defender Mathieu Debuchy to join French club St Etienne.
Update 12.28pm: Aubameyang's confirmed arrival is expected to be followed by Chelsea forward Michy Batshuayi heading to Dortmund on a loan deal, with the transfer window in Germany set to close at 5pm GMT.
During Wednesday morning, photos emerged on social media of Batshuayi undergoing what appeared to be a medical in Dortmund club shorts.
That move in turn should clear the way for Olivier Giroud to join Chelsea, with a fee of around £18million reported to have been agreed for the France forward who came off the bench in Arsenal's 3-1 defeat at Swansea on Tuesday night.
Following the loss in Wales, a video of Arsenal manager Arsene Wenger discussing the signing of Aubameyang was uploaded to the club's official website, posted to an article which looked back on the match, before being later removed.
Arsenal uploaded the wrong video on their link. Here is wenger confirming Aubameyang. This club can't do anything right LMAO 😂😂@fkhanage @hughwizzy @aliladiere @DarrenArsenal1 @AFCAMDEN @CarlBovis_AFC @MrArsenicTM pic.twitter.com/NsSpdCYC7S— Jiroo (@GadGooner) January 31, 2018
Arsenal confirmed the transfer just after 11.15am on Wednesday.
Update 12.08pm: Ki Sung-yueng has committed himself to Swansea's survival battle and insisted his West Ham suitors "are no better than us".
Press Association Sport understands the Hammers are keen on adding Ki and Swansea team-mate Alfie Mawson to their own Premier League relegation scrap before the 11pm deadline on Wednesday night.
But Swansea do not want to sell anyone and South Korean midfielder Ki - who is out of contract in the summer - plans to see out the season at the Liberty Stadium.
"I have already said my concentration is to survive in the league here and we are starting to play better," Ki said after Swansea moved out of the bottom three by beating Arsenal 3-1 on Tuesday night. "I want to be here and if Swansea want me to stay here, then I can help this team stay in the Premier League.
"It is a difficult time but I don't think I am going to leave. I don't need to move. I don't think West Ham are better than us."
Update 11.14am: Arsenal have completed the signing of Borussia Dortmund striker Pierre-Emerick Aubameyang for a club-record fee.
The Gunners announced the 28-year-old has signed a "long-term contact" at the Emirates Stadium.
The Gabon international, who scored 98 goals in 144 Bundesliga games for Dortmund, is a replacement for Alexis Sanchez following the Chilean's move to Manchester United.
Neither club confirmed the fee, but reports claimed Arsenal have paid around £56million for Aubameyang, whose arrival follows that of Henrikh Mkhitaryan from Manchester United.
It could also pave the way for the departure of Olivier Giroud, who has been linked with a move to Chelsea.
Original story (11am): 'Complicated' three-way deal edges closer; Mahrez linked to Man City
Manchester City could make a deadline-day transfer raid for Leicester forward Riyad Mahrez but look certain to have to stump up another club-record fee.
The Algeria international is understood to have been earmarked as a potential late target for the Premier League leaders after the blow of losing Leroy Sane to injury.
Reports on Tuesday claimed the Premier League leaders have had a £55million pound bid turned down, as the Foxes want £90million for Mahrez.
The Leicester winger has submitted a transfer request.
The 26-year-old would give City manager Pep Guardiola another attacking option as he chases glory in four competitions.
Arsenal manager Arsene Wenger accepts getting a deal for Pierre-Emerick Aubameyang pushed through ahead of the transfer deadline was always likely to prove "complicated".
Press Association Sport understands there is an agreement in principle between Arsenal and Borussia Dortmund for the Gabon forward, who was widely reported to have travelled to London on Tuesday to finalise a potential £60million switch.
However, there is the added issue of Dortmund first seeking assurances over a replacement - which could well be Michy Batshuayi from Chelsea.
Dortmund have agreed to sign Batshuayi on loan for the rest of the season, according to reports.
Chelsea are now looking to complete the final leg of the transfer triangle, by signing Olivier Giroud.
The French striker has been at the Emirates Stadium since 2012, but appeared to say goodbye to the Arsenal fans after last night's 3-1 defeat at Swansea.
Manchester City were extremely busy on Tuesday, signing defender Aymeric Laporte from Athletic Bilbao in a reported £57million deal before bringing in English midfielder Jack Harrison from sister club New York City.
Harrison joined City on a three-and-a-half-year deal before heading straight out on loan to Championship side Middlesbrough for the rest of the season.
Arsenal will miss out on Monaco midfielder Thomas Lemar.
The Ligue 1 champions' vice-president Vadim Vasilyev has reiterated that the France international - also a reported target for Liverpool - is not for sale this month.
"He will stay at Monaco until the end of the season 100 per cent," Vasilyev told French radio station RMC. "If Liverpool or Arsenal come with a big offer before the end of the market? This is not relevant."
Southampton's attempts to sign forward Quincy Promes are being met by resistance from Spartak Moscow, Press Association Sport understands.
It looks set to be a busy deadline day for Championship clubs after several managers vowed to dip into the transfer market following Tuesday night's fixtures.
Sunderland boss Chris Coleman said Ovie Ejaria will be joining his side on loan from Liverpool as a replacement for Watford-bound Didier Ndong after their 3-1 defeat to Birmingham, while Nottingham Forest boss Aitor Karanka said he would be adding to his squad following the 3-0 home defeat against Preston.
Jordan Hugill was left out of Preston's matchday squad against Forest after boss Alex Neil decided he was not in the right frame of mind amid speculation linking him with a move to Crystal Palace.