Network Rail is advising all passengers not to travel with all trains suspended between London Euston and Milton Keynes.
The emergency services are dealing with a line side fire caused after 25,000 volt overhead electric cables came down in Harrow.
All services to and from London Euston are suspended until further notice.
Trains are currently terminating north of the affected area, at stations such as Milton Keynes and Rugby.
Specialist Network Rail electrical engineers can assess damage caused to the cables and immediately start the repair once given the all clear by the fire service.
Trees involved in the fire can also then be cleared from the tracks.
James Dean, Network Rail’s West Coast South route director, said: “As predicted the extreme temperatures have impacted the overhead cables on the West Coast main line and all trains have had to be stopped at Harrow in North London.
“Please follow our ‘do not travel’ message today as journeys are being severely impacted. Once the emergency services give us the go ahead we will work as fast as we can to restore the railway lines. We’re sorry to people impacted and we’re working as fast as we can to get things back up and running.”
