Air passengers at two of the UK's largest airports are being hit by more disruptions caused by a problem with the country's air traffic control system.
It occurs after Heathrow and Gatwick have already suffered a day of flight cancellations and delays due to the storms that followed the heat wave this week.
The UK air traffic controller, Nats, said the situation would improve after solving the problem at the Swanwick Control Center.
Rail delays caused by extreme heat also began to decrease on Friday.
The Met Office said Thursday was potentially the hottest day in the United Kingdom after it received a provisional recording of 38.7C at the Cambridge Botanic Garden.
The reading will require "quality control and analysis" in the next few days, the forecaster said.
The official figures, which recorded 38.1C in Cambridge, already put Thursday as the hottest day recorded in July and the second hottest day in the United Kingdom.
& # 39; No idea what to do & # 39;
Mark Pickering was scheduled to fly to Singapore on his way to Sydney, but his Thursday afternoon flight from Heathrow was canceled due to "damaged" food caused by hot weather.
He waited at the airport until 02:00 BST on Friday morning, before arriving at a hotel and returning at 07:00 to try again.
"I still don't know where my bags are and I have no idea what to do when I finally arrive in Singapore because I lost my connecting flight for years," he said.
"When the flight was canceled last night, there was a line of approximately 300 to 400 people and nobody really knew what was going on. There was a person at the check-in desk."
Diane Styant, who booked to fly on the same flight to Singapore as Mr. Pickering, said he could not get any accommodation Thursday night.
She said she had to sit in a vertical chair in a cafeteria until 03:30, when she and her husband joined the departure row to make a new reservation.
After more than four hours, they managed to book another flight that left on Friday.
"Our beautiful annual vacation to visit our family living in Singapore has not had good results," he added.
Friday has been colder than Thursday, with temperatures ranging from 23C to 25C in most areas (73F to 77F), rising to 27C in the southeast of England.
During the day it is expected to be mostly dry, but rain is expected to develop in the northern and eastern areas of the United Kingdom during the night.
The interruption of flights on Friday was caused by storms throughout Europe.
A "technical problem" at the Swanwick air traffic control center on Friday morning also caused problems in Heathrow and Gatwick.
The UK air traffic controller said in the afternoon that he had "solved the problem enough to safely increase traffic flow rates and you will see an improved image for the rest of the day."
The European air traffic coordination agency, Eurocontrol, said the problem was caused by a "problem with radar screens."
Both Heathrow Airport and Gatwick Airport asked passengers to check with airlines before traveling.
British Airways said severe storms had caused "significant delays and cancellations in our operations inside and outside London."
The Independent travel editor, Simon Calder, said that "dozens" of the canceled flights were on easyJet and that there were "some very long delays on British Airways."
On Friday night, at London Liverpool Street station, passengers said on social media that all trains had been canceled, leaving them stranded.
Author Erica James described the scene at the station as "hell," while her train travel companion Ross Brereton warned others on Twitter To avoid the season "like the plague."
Elsewhere, there was a break in the Midland mainline between London St Pancras and Sheffield and in the services operated by Great Northern, Thameslink, East Midlands Trains and West Midlands Trains.
West Midlands Railway said night work to repair damage was still ongoing and many trains were starting out of place, causing cancellations and shorter trains.
Passengers using Eurostar services to and from Paris also face a "serious disruption" due to the problems of overhead power lines in the French capital.
Many operators said tickets for Friday could be used the next day, and offered compensation to season ticket holders who did not travel on Thursday.
Anthony Smith, executive director of the independent regulator Transport Focus, said: "All passengers who must not travel due to extreme weather must be entitled to claim compensation."