The extreme heat - which would be a record temperature in the UK - can have a particularly big impact on the rail network, as there is a risk of tracks buckling.
Speed restrictions have been put in place across London due to the Met Office’s first ever red weather warning for hot weather.
This has meant train companies have had to cancel services to allow longer journeys across the network.
In particular, Thameslink has been affected by the weather - and there will be no train services from King’s Cross or Moorgate on Tuesday.
The Gatwick Express is also not running.
All companies are advising passengers to check before they travel, as trains can get cancelled at the last minute.
Thameslink cancellations and delays
London King’s Cross, St Pancras, Finsbury Park & Moorgate (Monday only)
- There will be a very reduced service between Cambridge and London, a stopping service only.
- Services between Moorgate and Stevenage / Welwyn Garden City will be reduced, with two trains per hour on each route.
- Peterborough to Horsham services will not run south - only run between Peterborough and London King’s Cross.
- There will be no services between Finsbury Park and St Pancras, meaning no peak services between Sevenoaks, London and Welwyn Garden City.
London King’s Cross, St Pancras, Finsbury Park & Moorgate (Tuesday only)
- You cannot travel on Thameslink routes to or from London King’s Cross or Moorgate on Tuesday.
Bedford to London, Wimbledon, Sutton & Orpington (Monday and Tuesday)
- Bedford to London services will be reduced to two trains per hour.
- Luton to Rainham services will run, stopping at all stations between Luton and London.
- St Albans to Wimbledon / Sutton services will be suspended on Monday and Tuesday.
- There will be no peak Thameslink services to or from Orpington.
South of London (Monday and Tuesday)
- There will be no Thameslink service between Sutton and London, either via Mitcham Junction or Wimbledon.
- There will be two Thameslink trains per hour between London, Gatwick, and Brighton (not calling at Redhill).
- There will be no Thameslink service to/from East Grinstead, Orpington or Littlehampton.
- Thameslink services will run between Blackfriars and Sevenoaks only.
- The Gatwick Express will not run.
Southern Rail cancellations
- Southern services between East Croydon and London Bridge via New Cross Gate will not run.
- Southern services between Caterham and London Bridge via Tulse Hill will not run.
- Streatham station will have no service.
- There will be one Southern train per hour between Brighton and London Victoria.
- Bexleyheath Line: No evening services to and from London Cannon Street.
- Woolwich Line: Fewer services between London Cannon Street and Slade Green via Woolwich.
- Bromley South Line: Afternoon and evening journeys which start or finish at Bromley South will not run.
- Hayes, Grove Park & Sidcup lines: A small number of services to and from London Cannon Street on these routes will be cancelled in the afternoon.
South Western Railway cancellations
South Western Railway is advising passengers to use its journey planner tool to see if services are being delayed or cancelled due to the heat.
- Due to a safety inspection between Clapham Junction and Waterloo, delays are expected until 2pm on Monday.
Chiltern Railways cancellations
Chiltern Railways services are running from London Marylebone today, however the company has said there will be an amended timetable and extended journey times.
On Monday and Tuesday, there will only be one train per hour from Marylebone to Birmingham Moor Street, Oxford, Aylesbury (via High Wycombe) and Aylesbury Vale Parkway (via Amersham).
Greater Anglia cancellations
There are speed restrictions on the entire Greater Anglia network.
- A reduced service will operate on the Norwich-London Liverpool Street and Cambridge-London Liverpool Street main lines and on the Southend-London Liverpool Street route.
- There will be no services to and from London to Harwich-Manningtree, Braintree-Witham, Southminster-Wickford and Clacton/Walton/Colchester Town-Colchester branches.
- The Stansted Express service between Stansted Airport and London is likely to remain half-hourly but journeys will take longer.
C2c has said that its trains will be delayed and cancelled today and tomorrow - and customers should check before travelling.
So far on Monday, more than 20 trains running to and from London Fenchurch Street have been cancelled.
Due to a fire in Upminster next to the railway, the line between Ockendon and Upminster has been closed.
Greater Anglia are accepting c2c tickets and a rail replacement bus service is running.
Can I get a refund?
Customers can use tickets dated Monday and Tuesday to travel with Southern, Gatwick Express, Thameslink or Great Northern on Wednesday and Thursday
If Thameslink customers are unable to complete their journey as their train is cancelled, they can apply for a refund.
South Western Railway is allowing people with tickets for travel on its services on Monday and Tuesday to travel on Wednesday and Thursday instead.
Chiltern Railways is offering the same service.
Southeastern says that customers who choose not to travel are eligible for a free refund.
If you have a monthly or longer season ticket and choose not to travel, you can claim Delay Repay for the hot weather dates.
Southeastern customers can also use tickets for Monday and Tuesday on Wednesday and Thursday.
C2c is asking customers to claim through delay repay if their services is delayed by more than 15 minutes or cancelled.
Greater Anglia says customers with tickets for Monday and Tuesday will be able to use their tickets on another day or apply for a free refund.