Jeremy Corbyn has said Labour faces a challenge on a "historic scale" if it is to regain power in the General Election on June 8 after the party suffered heavy losses in local council polls.
The Conservatives made sweeping advances across the UK, gaining more than 500 councillors, winning tightly-fought battles for elected mayors in the West Midlands and Tees Valley and forcing Labour into third place in its former stronghold of Scotland.
The emphatic victories, fuelled by a collapse in the Ukip vote as anti-EU supporters flocked back to the Tories, set the scene for a substantial increase in Theresa May's House of Commons majority if repeated in June.
The Prime Minister sought to fight any complacency in Tory ranks, insisting she was "taking nothing for granted" in an election she hopes will strengthen her hand in Brexit negotiations with Brussels.
Labour tried to play down the significance of a bruising set of results which saw it forfeit more than 300 council seats, lose control of Glasgow for the first time in around 40 years and suffer reverses in Welsh strongholds.
Mr Corbyn described the results as "mixed" and insisted they were "closing the gap" on the Conservatives, but acknowledged they faced a huge challenge.
"We have five weeks to win the General Election so we can fundamentally transform Britain for the many not the few," he said in a statement.
"We know this is no small task, it is a challenge on an historic scale.
"But we, the whole Labour movement and the British people, can't afford not to seize our moment.
Liberal Democrats had a mixed election, failing to break through against the Tories in the south-west England battleground but making advances in some General Election target seats like Eastleigh and Wells.
Speaking during a visit to a factory in Brentford, west London, Mrs May said: "I will not take anything for granted and neither will the team I lead, because there is too much at stake.
"The reality is that today, despite the evident will of the British people, we have bureaucrats in Europe who are questioning our resolve to get the right deal.
"And the reality is that only a General Election vote for the Conservatives in 34 days' time will strengthen my hand to get the best deal for Britain from Brexit."
As Ukip shed 109 councillors while holding a solitary seat in Lancashire, leader Paul Nuttall said the party was "a victim of its own success" over Brexit.
Losing seats to Tories was "a price Ukip is prepared to pay" for the prize of leaving the EU, he said.
The results sparked a furious reaction from the party's former donor Arron Banks, who said Ukip was "finished as an electoral force" under its current leadership and needed "a strategic bullet to the back of the head".
Conservatives won the West of England metro mayor contest, while Labour claimed the mayoralties in Greater Manchester, won in the first round of voting by former cabinet minister Andy Burnham, Liverpool, Doncaster and North Tyneside.
A day of triumph for Tories and misery for Labour saw:
:: Severe blows for Labour in the south Wales valleys, with independents taking control of Blaenau Gwent and Merthyr Tydfil.
:: Labour held key Welsh councils in Cardiff, Swansea and Newport, but lost control of Bridgend, the stronghold of First Minister Carwyn Jones, which Mrs May had targeted with a campaign visit.
:: Former John Lewis managing director Andy Street won the West Midlands mayoralty by less than 4,000 votes over Labour's Sion Simon, after the second preferences of other candidates were redistributed.
:: In a highly symbolic victory, Conservative Ben Houchen became the first elected mayor of Tees Valley, in Labour's north-eastern heartland, beating Labour's Sue Jeffrey by more than 2,000 votes in the second round of counting.
:: Victory for Conservative Tim Bowles over Labour's Lesley Mansell in the run-off for the West of England metro-mayoral post, covering Bristol and Bath.
:: A Ukip wipeout in former stronghold Lincolnshire, where Conservatives took overall control.
:: Conservatives also took control of Cambridgeshire, East Sussex, Norfolk, Suffolk, Monmouthshire, Isle of Wight, Derbyshire, Gloucestershire, Warwickshire and Lancashire. In Derbyshire, the Tories took 19 seats from Labour as the county passed directly from one party's hands to the other.
Shadow chancellor John McDonnell insisted Labour's vote "held up" in areas of Wales where Mr Corbyn had campaigned, and said there were signs voters were developing more confidence in the party leader as he had the chance to get his message across.
Speaking during an early-morning round of interviews, he said: "So far, it's been a tough night, but it hasn't been the wipeout that people expected."
Defence Secretary Sir Michael Fallon said Conservatives were winning support from former Labour voters because of Mr Corbyn's "feeble" leadership.
Lib Dem leader Tim Farron claimed strong performances in target areas like St Albans, where he addressed activists, meant his party was on track to more than double its presence of nine MPs at Westminster.
With full results declared by all 88 councils holding elections in England, Scotland and Wales, the Tories had control of 28 authorities, including 11 gains.
Mrs May's party had 1,900 councillors, a net gain of 558, while Labour had 1,151, a net loss of 320.
Among the authorities which had declared their final results, Labour had control of nine councils and had lost control of seven.
The Lib Dems had 441 councillors, a net loss of 37; the Scottish National Party had 431, up 31; Plaid Cymru was on 202, a net gain of 26 and the Greens were on 40, a net gain of six.
A BBC projection of the overall national vote share put Conservatives on 38% (up 13 points since 2013), Labour 27% (down two), Liberal Democrats 18% (up four) and Ukip 5% (down 18).