Several fast trains from London Paddington have been cancelled by First Great Western today, after Network Rail imposed speed restrictions to prevent track from buckling in the heat.
Six services out of 1,520 running today have been affected so far.
A spokesman said: "First Great Western is advising passengers that due to the anticipated hot weather Network Rail is to impose a speed restriction in the London Thames Valley area to protect track points on Tuesday 30 June.
"As a result there will be no fast trains between between London Paddington and Bourne End or Henley-on-Thames from 12:00 until 20:00."
Network has told train companies to slow down at vulnerable locations as tracks could buckle.
With temperatures set to soar into the mid 30s this week, train companies are warning that the heat could interfere with Britain's rail network.
Commuters are being told to check their journeys ahead of time for ristricted services, as the heat is predicted to get so fierce that it could buckle rails.
"As rails are made out of steel, they expand as they heat up and are subject to strong compression," Network Rail said on its website.
"This expansion has to be managed to reduce the risk of track buckling. If the track does buckle, the line must be closed and the track repaired before services can resume, causing considerable disruption."
First Great Western services in the Thames Valley area will be subject to speed restrictions from midday to 8pm today - meaning there will be fewer fast trains.
Network Rail today announced that its plans to improve Britain's rail system were unachievable, and now many major projects are on hold, or even cancelled.
ITV News correspondent Romilly Weeks reports:
On today's disappointing statement on Network Rail, ITV News consumer editor Chris Choi said:
"Network Rail today refused my repeated interview requests. Its underestimate of costs and over-promise of modernisation has led today to its chairman stepping down.
"But the biggest casualty is likely to be passenger trust in railway promises - that seem as unreliable as too many of its services."
Network Rail signed up to "highly ambitious" targets which "proved too optimistic", according to the Rail Delivery Group.
Michael Roberts, the director general of the group, which represents operators, said: "Britain’s railway is carrying record numbers of passengers and freight and in some places is virtually full to capacity, making continued investment crucial."
He added: "The majority of proposed enhancements will still go ahead, and it is important that the industry, government and regulator learn from this situation to ensure we do better in delivering the future improvements the railway needs.”
- Background: Unions suspend planned Bank Holiday strike
Talks to avert a rail strike have resumed today.
Members of the Rail, Maritime and Transport (RMT) union at Network Rail are due to walk out for 24 hours from next Thursday evening and 48 hours the following week, threatening travel chaos after a new pay offer was rejected.
- Read more: Rail workers to strike next month
The planned Bank Holiday rail strike has been cancelled after the RMT union joined the TSSA in suspending industrial action.
The TSSA - one of the two rail unions - planning strike action over the Bank Holiday has suspended the action after receiving a new pay offer from Network Rail.
Members of the Transport Salaried Staffs Association were due to walk out for 24 hours from 5pm on Monday.
But after talks with Network Rail, Manuel Cortes, general secretary of the TSSA, confirmed that the union had suspended the action,
The RMT, which represents most Network Rail employees, is still to announce if the strike is going ahead.
ITV News' Business Editor Joel Hills has learned that Network Rail has tabled a new pay offer to RMT, Unite and TSSA unions in a bid to avert next week's planned strike.
Breaking: Network Rail has tabled a new pay offer to RMT, Unite and TSSA unions. Talks continue but I'm told "deal could be close".