The Prime Minister has warned train operator London Midland that action may be taken if its service does not improve.
Trains between St Albans Abbey Station and Watford Junction have been cancelled regularly due to a lack of staff, forcing commuters to use replacement bus services.
In an interview with the BBC, David Cameron said that the company had been put on a "warning list".
He 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."
However, Mr Cameron said that he was confident that a new timetabling system, which came into operation yesterday, will help the situation.
Richard Harrington, the MP for Watford, met with the company’s bosses at the end of last month following weeks of disrupted services.
On hearing that Mr Cameron has shown concern, Mr Harrington said: "I have spoken to him personally about this.
"I’m not prepared to listen to any more excuses."
Daniel Scaife, external relations manager at London Midland, said: "Sunday's timetable change has enabled us to identify a number of operational efficiencies.
"Specifically, we have been able to make some changes to the way in which work is diagrammed, or rostered.
"This process has enabled us to address the temporary driver shortfall. We apologise to anyone who has been impacted by the recent problems.
"A number of trainee drivers finish their training very soon which will assist in dealing with short notice unavailability, such as on the day sickness, which all operators face.
"We are also continuing our recruitment and training process to add further resource resilience."