The Met Office issued a first-ever red extreme heat warning for Monday, July 18, and Tuesday, July 19, including the capital.
Temperatures could hit 40C today, with it highly likely that the mercury will break previous records.
Transport for London (TfL) advised Londoners only to travel if absolutely necessary yesterday and today.
And already the hot weather is crippling the Tube network, with the Hammersmith and City line completely suspended due to the heat.
As of 9am on Tuesday, these are the TfL services which are already suspended or running a reduced service.
Which TfL services are cancelled or delayed?
- Central line: Severe delays on the whole line due to heat related speed restrictions.
- District line: Severe delays on the whole line due to heat related speed restrictions.
- Elizabeth line: Minor delays between Liverpool Street and Shenfield and between Paddington and Heathrow/Reading due to heat related speed restrictions.
- Hammersmith & City line: Entire line suspended.
- Jubilee line: Minor delays due to heat related speed restrictions.
- Metropolitan line: No service Baker Street to Aldgate due to heat related speed restrictions, and severe delays on the rest of the line.
- Piccadilly line: Minor delays due to heat related speed restrictions.
- No service from Willesden Junction to Richmond due to heat related speed restrictions.
- No service from Romford to Upminster due to heat related speed restrictions.
- Minor delays from Watford Junction to Euston; Clapham Junction to Stratford; and Liverpool Street to Chingford, Cheshunt and Enfield Town due to heat related speed restrictions.
- No service between Gospel Oak to Barking Riverside while we fix a faulty train at Wanstead Park.
TfL travel advice
TfL has advised customers to only travel if essential on Monday and Tuesday, due to the high temperatures forecast.
Temporary speed restrictions will need to be introduced on parts of the Tube and rail network as a precaution, which has led to reduced service and journeys taking longer than usual.
Track temperature checks will be carried out regularly across to ensure they remain safe and the track is not at risk of bending or buckling.
Andy Lord, chief operating officer, said: "Due to the exceptionally hot weather that is expected, customers should only use London's transport network for essential journeys.
“If customers do need to travel, they should check before they travel as we are expecting there to be some impact to Tube and rail services as a result of temporary speed restrictions we will need to introduce to keep everyone safe.
“It is also vital that customers always carry water at all times with them when travelling."