Luton Airport was forced to suspend flights because of a defect in the runway caused by high temperatures - with the heatwave gripping the UK set to get worse.
The airport said: "Following today's high temperatures, a surface defect was identified on the runway."
All incoming and outgoing flights were suspended shortly after 3pm.
Departures were given the green light at 5.40pm with arrivals restarting shortly after 6pm.
Sky News understands that some flights were diverted to nearby airports, with Biggin Hill among them, according to the Flight Radar website.
The RAF also halted flights in and out of its largest air base in the UK, Brize Norton, because the "runway has melted", according to a military source.
Amid warnings that lives are at risk, a national emergency has been declared by the UK Health Security Agency and the Met Office has issued its first-ever red warning for extreme heat.
The heatwave is set to peak on Tuesday, with Monday set to see maximums of around 39C (102.2F) in parts of the West Midlands during late afternoon, according to the Met Office.
Travel delays as tracks 'bend and buckle'
People are being urged to avoid all non-essential travel and warned of major disruption to journeys by train and car.
Kevin Groves from Network Rail says speed restrictions covering much of the network were due to come into force from midday, due to the risk of tracks buckling in the heat.
Speeds will be slowest in the east of the country where temperatures are likely to be too high for infrastructure there to withstand. Trains between Leeds and London will be running at half their normal speed, doubling the journey time between the two cities to five hours.
The network tweeted that the extreme heat is causing the overhead line equipment that powers electric trains to sag.
In London, on the Transport for London network, parts of the Overground, Metropolitan and Hammersmith and City lines are suspended due to the heat, with severe delays affecting large parts of the rest of the infrastructure under its umbrella. TFL has urged people in the capital not to travel on its network today and tomorrow, with temperatures of well over 30C reported on the Tube.
At Vauxhall, on the rail network in the capital, Network Rail Wessex reported a kink in the tracks as the metal reached 48C, prompting railway staff to paint them white to reflect the sun and stop them heating further.
Reduced services are running on most long-distance rail services, including LNER, East Midlands Railway and Lumo and Scotrail and Transport for Wales are also reporting delays and changes.
Avanti West Coast, which operates between Euston and Scotland, via the West Midlands, has suspended ticket sales for the rest of the day and tomorrow.
Sky's Becky Johnson at Birmingham's New Street Station says all the services into and out of the station are reporting delays.
Meanwhile, motorists have been advised to try to make their journeys outside the hottest periods of the day, particularly if they have older cars.
The RAC has warned more drivers will need help as cars overheat, and asked their customers to question whether they really need to make the journey in the first place.
The disruption taking place has led the government to hold another emergency Cobra meeting to discuss the response to the heat, with Cabinet Office minister Kit Malthouse taking the chair amid criticism from opposition politicians who have said Boris Johnson has "clocked off" by not taking a lead on the issue.
Hospital appointments cancelled
Some hospitals have taken the decision to cancel routine appointments and surgeries due to the risk to both patients and staff due to the predicted extreme heat.
With ambulance wait times already on the rise, there have been warnings that things will only worsen during the heatwave, with patients potentially stuck in hot vehicles for hours while waiting for hospital admission.
The NHS's top doctor said that while the overwhelming majority of NHS services continue to run, the situation is being monitored closely.
NHS Medical Director Professor Sir Stephen Powis said: "Thanks to the hard work and innovation of NHS staff the overwhelming majority of NHS services are continuing to run despite the unprecedented heatwave.
"We know that some people, particularly the elderly or vulnerable, may have been unable to attend appointments because of the hot weather or disruption to other services, such as public transport.
"With tomorrow set to be as hot, if not hotter, than today, we are monitoring the situation closely and people should take simple, common sense steps to stay well such as drinking enough and staying out of the sun."
East Midlands Ambulance Service Director of Operations David Williams warned patients they may face "an extended wait" for an ambulance and urged people to call 999 only as "a last resort".
Contingency support, such as more call handlers and extra working hours, have been put in place for ambulance services while the chairman of the NHS Confederation said hospitals are going to be "really, really pushed" over the next few days.
Emergency services have urged people to be careful when trying to stay cool after the body of a boy was recovered in Salford Quays, Greater Manchester, in the early hours of Sunday morning.
School closures and sports days cancelled
Some schools in several counties, including Nottinghamshire and Hampshire, have confirmed they will close, and the NEU teaching union has said it will support headteachers taking that decision.
Some 53 schools in Buckinghamshire have closed and reverted to remote learning.
In Norfolk, North Wootton Academy near King's Lynn has closed due to a low water supply.
Many others are taking contingency measures, with PE lessons cancelled and pupils being allowed to leave early.
School sports days scheduled to take place during the hot weather are also being postponed or cancelled.
Some schools have said they will arrange staffing to keep sites open for parents unable to find alternative childcare at short notice.
Shadow education secretary Bridget Phillipson said children should still go to school.
"I am sure that headteachers will be doing everything they possibly can to make sure their schools are kept as cool as possible, that children are kept out of the hot sun and are kept indoors as much as possible," she told Sky News.
"I think children have missed out quite a lot already in terms of their education, and it's right for them to be there."
Some southern nurseries will be restricting their hours on the hottest days following Public Health England's guidelines for supporting children in Early Years.
The majority of UK schools are set to break up for the summer holidays later in the week.
Some shops and salons around the country have also advised they will be closing or shutting early on Monday and Tuesday.
Unprecedented weather conditions have led to many weekend events - including dog shows and summer fetes - being called off, as well as larger scale events at the start of the week.
The British Horseracing Authority (BHA) has called off five meetings - scheduled fixtures at Beverley and Windsor on Monday and all-weather cards at Chelmsford and Wolverhampton on Tuesday, along with Southwell's jumps fixture.
Meanwhile, water companies have been seeing "unprecedented peak demand" during the extreme heat.
Householders are being asked to "carefully consider" their water usage, and not to waste it in the high temperatures.
Water UK said the "most intense demand" was across the south of England but that "pretty much all companies are seeing elevated demand at the moment".
The Met Office's chief meteorologist, Paul Davies, said the "unprecedented heat" is something the UK is going to have to get used to as it could be a "regular occurrence by the end of the century".
He told Sky News: "I've never seen the charts that I've seen today. The speed at which we are seeing these exceptionally high temperatures is broadly in line with what we are saying, but... to see the brutality of the heat that we are expecting tomorrow is quite astounding. It does worry me a lot."