Hundreds of flights at London's Heathrow and Gatwick airports have been delayed as the UK's air traffic control service warned disruption was expected to continue into Sunday.

More than 900 inbound and outbound flights at the two travel hubs have been delayed for up to 11 hours and dozens cancelled, according to tracker FlightAware, many due to thunderstorms.

NATS, the UK's air traffic control service, said adverse weather conditions meant aircraft needed to "avoid affected areas and be spaced further apart" and this would impact flights for the "remainder of Saturday and overnight".

Gatwick and Heathrow advised passengers to check the status of their flights with airlines before travelling.

More than 474 flights arriving at or departing from Heathrow were delayed and another 439 in and out of Gatwick on Saturday, according to FlightAware.

Delays to affected flights vary. Heathrow's arrival board showed a British Airways flight from Santiago, Chile, that was due to land at 10:00 BST was not expected to arrive until 21:00.

Europe-wide aviation agency Eurocontrol mapping showed the most severe air traffic control delays were in airspace between south-east England and north-western Europe, where the storm clouds were located.

However, other UK airports remain largely unaffected.

In an updated statement on Saturday afternoon, NATS said: "Severe thunderstorms across the south east of England and Europe continue to impact flights, and this is expected to continue for the remainder of today and overnight.

"To ensure the safety of the travelling public, aircraft need to avoid affected areas and be spaced further apart, which limits the number of flights that can operate safely.

"We understand disruption is frustrating, but we're working closely with airlines and airports to reduce disruption as much as possible.

"Passengers should continue to contact their airline for the latest information on their flight."

NATS added that the forecast in UK airspace looked generally fine on Sunday, but that further thunderstorms were forecast in Europe, which may have a further impact on travel routes across the continent.

The thunderstorms developed overnight following a record-breaking heatwave, with the UK's hottest ever June high of 37.3C recorded in Suffolk on Friday.

Eurocontrol said a "broad area of hot, unstable air" stretching from northern Spain to southern Sweden was likely to see further storm-cloud development overnight, adding that there was "a large degree of uncertainty" over when and where they would develop.

Air traffic control mapping showing airspace delays as of 12:50 BST, with delays of less than 15 minutes in blue and more than 45 minutes in red

Passenger Adam Joseph, 29, told BBC News that he had been stranded at Venice airport in Italy without air conditioning because his flight to Gatwick had been delayed by four hours and counting.

He was due to depart Venice at 12:30 local time but said the plane had not yet left London on its outbound leg.

"We could've stayed at the hotel for another three to four hours," Joseph said.

"We are also being told that even in the event of a four-hour-plus delay, because of an air traffic control restriction, we will not be entitled to compensation.

"I've had to give up my chair to a family with a pregnant mother.

"People are very angry. We have had no communication from [BA] whatsoever."

BA apologised for the inconvenience, adding: "Like other airlines, we've had to make some adjustments to our schedule today due to air traffic control restrictions caused by adverse weather conditions affecting parts of UK airspace."

Meanwhile, Easyjet passenger Will Poole told the BBC he had cancelled his holiday to the Greek island of Skiathos after his flight from Gatwick was cancelled and moved to Bristol.

He said he boarded shortly after 05:00 on Saturday but was brought back to the gate at 11:00.

Poole said he "was eventually offered a complimentary snack and drink after five hours and an email with a £6 voucher to spend in Gatwick".

Poole, who booked with Easyjet Holidays, told the BBC he was then offered a replacement flight at 07:00 on Sunday from Bristol Airport, more than 100 miles away.

"In the email it said 'details to follow' about them arranging a transfer, but no mention of a hotel," he said.

"Easyjet staff in the terminal were not very helpful, saying we needed to speak to someone at Easyjet Holidays.

"I got through eventually and decided to cancel the holiday as they couldn't get us on another flight within 12 hours."

Poole said passengers understood the situation was out of Easyjet's control, but "you'd like to think" the replacement flight would have been scheduled from Gatwick. He said he has been promised a refund within 28 days.

Easyjet said in a statement that Poole's flight was unable to operate due to thunderstorms restricting the number of arrivals and departures at Gatwick.

"We're really sorry that Mr Poole had to cancel his holiday with us due to weather disruption affecting his flight," the spokesperson said, confirming that he would receive a full refund as the company could not find an alternative flight suitable for his travel plans.

It added that affected passengers were being given options including refunds, re-bookings, hotel accommodation and meals where required.

A London Gatwick Airport spokesperson said "temporary air traffic restrictions" had been put in place "due to ongoing thunderstorms across the network last night".

A Heathrow Airport spokesperson said "adverse weather conditions" had led to "temporary air traffic restrictions impacting some flights".