Passengers are facing delays and cancellations after train drivers stopped working overtime.
A statement issued by London Midland said it was due to an agreement relating to drivers working overtime not being renewed by Aslef.
It added there would be a "significant number" of trains cancelled on Friday and Saturday.
The service operates throughout the Midlands connecting to London and to the North West.
London Midland said it was doing all it could to minimise potential disruption and to resolve the issue.
A spokesman for Aslef, the train drivers union, confirmed the train drivers' action.
He said there had been problems over renewing the agreement which had still to be sorted out.