London Midland said services in and out of Euston on the line are expected to be severely disrupted.
Emergency services are dealing with an incident on the line between Northampton and Euston. Hemel Hempstead station remains closed until further notice.
Virgin Trains running along the line are also facing the same disruption.
London Midland’s head of west coast services, Steve Helfet said: “We have arranged for London Midland tickets to be accepted on all reasonable routes so the advice is to travel on the best train to get you closest to your desired destination. We are also arranging road transportation between Northampton and Wellingborough so that passengers can travel on East Midland Trains from St Pancras.”
London Midland tickets can be used on the following services:
Chiltern Railways between Birmingham Snow Hill and London Marylebone
Thameslink between London St Pancras to St Albans & Bedford
First Great Western between London Paddington, Reading and Worcester
Cross Country between Leicester, Tamworth, Reading to Birmingham
Virgin Trains on all reasonable routes
Transport for London Buses