Rail passengers travelling in or out of London Euston have been advised not to travel.
Damage to overhead cabling means no West Coast main line services will arrive or depart until at least midday, Network Rail said.
Passengers using Virgin, London Midland and London Overground services will be affected and delays are expected throughout the day.
Underground trains from Euston will run as normal, Network Rail said.
Services between London Euston and the following stations are affected:
- Birmingham New Street
- Glasgow Central
- Liverpool Lime Street
- Manchester Piccadilly
- Milton Keynes Central
- Watford Junction
“If you must travel, please check the journey planner to make sure the line has reopened before starting your journey,” Network Rail said.
“Trains that do run are expected to be busier than normal, so travel should be considered for later in the day or on Monday where possible.”
Tickets valid for travel on Sunday would be accepted on Monday, it added.
The cause of the damage, which happened on Friday near Wembley in north-west London, is not yet known, it added.
Engineers were working “as quickly as possible”, a Network Rail spokesman said.
He said the company was “unsure at this stage of the cause of the damage”.