Rail regulators today warned Network Rail that it must improve punctuality and reliability on train services in Oxfordshire, the Thames Valley and the West of England.

Passenger services operated by First Great Western in these areas have been dogged by poor punctuality for several years and are among the worst-performing in Britain.

After repeated complaints from passenger groups, the Office of Rail Regulation is closely monitoring FGW's performance, amid calls for the company to be stripped of its franchise.

The ORR said that in the latest performance figures, only 83 per cent of FGW services were on time, compared with a national average of more than 90 per cent.

And at the turn of the year, only 64 per cent of FGW's peak-hour commuter services in the Thames Valley were on time.

Michael Lee, the ORR's director of access planning and performance, said: "Everyone agrees that the performance of services on Network Rail's western route has been poor for far too long. The cycle where performance takes a step forward, only to take another step back, is not delivering the necessary improvements."

In cases of prolonged poor performance on a particular part of the network, ORR can investigate whether the failure to meet targets constitutes a breach of Network Rail's licence.

Mr Lee added: "If targets for this route are not met in the near future, then the consequences for Network Rail could be severe.

"I do not propose, at this stage, to look into whether it is in breach of its licence, but all parties should be clear that this is an option that may be open in the future."