Millions more people are working from home to avoid severe disruption to transport networks caused by soaring temperatures.
Road traffic and public transport usage dropped on Monday after people were urged 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.
⚠️ Due to the red weather warning we must close the East Coast Main Line for all locations between London King’s Cross and York and Leeds on Tuesday afternoon.
We are warning you to not travel on Tuesday:
— Network Rail (@networkrail) July 17, 2022
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.
Monday 18 and Tuesday 19 July
Only travel if essential due to the high temperatures forecast
💦 Stay hydrated🤒 Seek assistance if you feel unwell🤳 Check your journey before you travel
— TfL – keep hydrated 💦👍🏿 (@TfL) July 18, 2022
“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 were expected to soar into the high 30s on Monday.
Train speed restrictions imposed by Network Rail to reduce the chances of tracks buckling in the heatwave caused delays and cancellations.
Many operators are running a heavily reduced timetable on Monday and 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.
He told Sky News: “Certainly later on today that (buckling) is a strong possibility, which is why, from about midday today through till 8pm tonight, there will be large swathes of England and Wales that will have emergency heat-related speed restrictions placed on the rail network.”
Some crucial, life-saving advice from our partners to keep you safe during the heatwave.
— Met Office (@metoffice) July 18, 2022
Jake Kelly, also from Network Rail, warned of travel disruption across the country.
He 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.