A major airport runway has been closed amid sizzling temperatures, as extreme heat saw people steer clear of roads and public transport to avoid serious travel disruption.

Engineers were called out to London Luton Airport to look at what was described as a “surface defect” on the runway.

Footage showed a large rectangular-shaped area on the tarmac being examined by people in high-vis vests on Monday afternoon.

A statement from the airport said: “Following today’s high temperatures, a surface defect was identified on the runway.

“Engineers were called immediately to site and repair works are currently in progress to resume operations as soon as possible.

“We would like to apologise for the inconvenience caused.”

Meanwhile, flying activity was also halted at RAF Brize Norton, with Sky News reporting that part of the runway had melted.

An RAF spokesman said: “During this period of extreme temperature flight safety remains our top priority, so aircraft are using alternative airfields in line with a long established plan.

“This means there is no impact on RAF operations.”

The mercury reached 36.1C at Brize Norton and 36C at Luton, the Met Office said.

Meanwhile, millions more people were working from home, heeding warnings to avoid unnecessary travel.

Network Rail said the number of passengers using major stations across Britain on Monday was around 20% down on a week ago.

Location technology firm TomTom said road congestion at 9am was lower in most UK cities than at the same time last week.

In London, congestion levels fell from 53% on July 11 to 42% on Monday.

In Birmingham they were down from 46% to 43%, in Manchester they decreased from 45% to 37%, and in Glasgow they dropped from 17% to 12%.

The figures reflect the proportion of additional time required for journeys compared with free-flow conditions.

Transport for London (TfL), which advised people to “only travel if essential”, said around 1.06 million entries and exits were made by London Underground passengers up to 10am on Monday.

This is down 18% compared with the same period last Monday.

Some 1.07 million bus journeys were made up to 10am, a 10% decrease week on week.

TfL said: “Ridership on Monday is typically lower than other days of the week on public transport and is therefore likely to be a good indication of where people are working from home.

“Typically, TfL also sees a small reduction in ridership at this time of year as schools enter their last week of term and people begin to go on holiday.

“However, the recent high temperatures have led to more of a reduction than would have been expected before our travel advice was issued to only make essential journeys during this extreme hot weather.”

Temperatures had risen to 38.1C in Santon Downham, Suffolk, by 4pm, making it the hottest day of the year, and temperatures topped 37C in a number of other places.

Train speed restrictions imposed by Network Rail to reduce the chances of tracks buckling in the heatwave caused delays and cancellations.

Many operators were running a heavily reduced timetable on Monday and will do so on Tuesday, including Avanti West Coast, CrossCountry, Great Northern and Thameslink.

LNER will run no trains from south of York and south of Leeds to London King’s Cross on Tuesday.

Kevin Groves, chief spokesman for Network Rail, said journeys which typically take two hours could take “more than four hours” as emergency measures have been brought in to prevent trains derailing.

Jake Kelly, also from Network Rail, warned of travel disruption across the country and told BBC Radio 4’s Today programme the return of normal services on Wednesday “will depend on the damage that the weather does to the infrastructure”.

Council gritters were on stand-by to spread light dustings of sand on melting roads.

The RAC anticipated that the number of vehicle breakdowns on Monday and Tuesday could be up to a fifth higher than normal.

Spokesman Rod Dennis said the increase in callouts will “put pressure on all breakdown services” as he advised drivers to carry an “emergency kit” such as water, non-perishable food, sun protection and any medication required.