London Midland warned by PM over cancellations
Action may be taken against London Midland if the train operator does not reduce the number of cancellations, the prime minister has said.
A shortage of drivers has caused the cancellation of more than 950 services since 14 October.
A new timetable was introduced on Sunday but three services were affected on Monday morning because of a lack of staff, according to the firm's website.
David Cameron said the company was on a "warning list".
Mr Cameron, in an interview with BBC WM, said: "It's not acceptable what's been happening.
"Under the franchising system if there's unacceptable service then action can be taken.
"It has in the past and it would be in the future if things don't pick up."
The prime minister said he was confident the new timetable would help address the issue.
The company has a number of trainee drivers who are due to finish their training soon.
London Midland's operations and safety director, Wallace Weatherill, said he could not rule out further delays or cancellations.
"There's never a guarantee when you're dealing with people and you're running over 1,300 trains a day," Mr Weatherill said.
"But what I can guarantee is there is no shortage of focus in making sure that every single one of our trains is covered so people can get to work and do what they need to do.
"When you have short-term sickness it's sometimes very difficult to recover from that."
Mr Weatherill said the likelihood of cancellations had "significantly" reduced because of the recent timetable change and the support of staff.