Update 5.35pm: Aer Lingus have said their services to Lisbon are operating as normal today, although their flight yesterday to and from the airport was affected.
The airline said: "Due a fuelling issue at Lisbon airport Aer Lingus flight EI486 from Dublin to Lisbon was rerouted to Faro yesterday evening and landed at 21.50 local time. Guests were accommodated by coach that evening to Lisbon.
"Inbound flight EI487 from Lisbon to Dublin was subsequently cancelled yesterday evening and guests due to travel were offered hotel accommodation and rebooked onto the next day’s flights.
"We apologise to all affected guests for the inconvenience caused which was due to circumstances outside our control. Today’s Dublin-Lisbon services are operating as normal."
Update 11.50am: Ryanair have released a statement after some customers were left stranded at Lisbon airport after a fuel pump failure.
The statement said: “Due to a fuelling system outage at Lisbon Airport last night (10 May), we regret that we were forced to cancel a number of flights to/from Lisbon.
"All affected customers were provided with refreshment and accommodation vouchers, were contacted by email and SMS text message and advised of their options of a full refund, a free transfer onto the next available flight, or a free transfer onto an alternative flight route, in full compliance with EU261 legislation.
"Ryanair sincerely apologised to all customers affected by this disruption, which was entirely beyond our control.”
Earlier: Tourists have been left stranded at a major European airport after a fuel pump breakdown grounded flights.
Passengers described seeing people sleeping on the terminal floor at Lisbon airport in Portugal after the system failure on Wednesday left thousands of people unable to get home.
Ryanair was among the airlines affected, along with the Portuguese national carrier TAP, which was warning customers that delays would run into Thursday.
Ella Sykes, 27, from Leeds, said the scene in the terminal was "absolute carnage" when she and a friend arrived at the airport at 2pm for a 5.30pm Ryanair flight to Manchester after a short break in Lisbon.
Speaking to the Press Association on Thursday morning, she said: "We have been here all night.
"It's 17 hours now and we still haven't had any information from Ryanair. We have given up on the queues. We've booked a flight from Faro tomorrow morning.
"We are exhausted and we've had to fork out for accommodation overnight as well. And I was meant to be at work this morning."
She said the only information she had received about what was happening had come from the media, adding: "The airport was absolute carnage. There were far too many people in the departure lounge so they stopped letting people in."
Other travellers took to social media to vent their frustration.
Priyank Lakhia, from London, said: "It's a mess at Lisbon Airport, flights cancelled and barely any staff on ground. Wife still waiting for updates."
Lisbon airport flights blocked due to a fuel system problem. Only one??!!!! Why can not come the flights with enough fuel to return??? pic.twitter.com/hXkqApHWXJ— Juan Salvador (@JuanSUbeira) May 10, 2017
Jonathan Millar, also from London, said on Wednesaday night: "Lisbon airport...what a mess. No fuel, no flights. Tracker boards haven't been updated since 2pm. Utter chaos."
On its Facebook page, the airport said there had been problems with the supply of fuel to aircraft, but it hoped flights would return to normal during Thursday.
Spaniard Javier Rayon wrote: "There are thousands of abandoned users in the airport, with no food, no water, no options to sleep and no options to flight (sic) even tomorrow. Disastrous management of the situation."