Delays to rail services in Oxfordshire, the Thames Valley and West of England are "a cause of serious concern", the rail regulator has said.

Train punctuality nationally was improving, but there was still a lot of hard work to be done on routes from London Paddington, a report about Network Rail from the Office of Rail Regulation added.

The report said Network Rail's performance on its western routes "remains poor" and delays were worse than the levels projected in a joint improvement plan agreed with train operator First Great Western.

It said Network Rail had been warned unless its performance improved in the last three months of this year, enforcement action would be considered.

Of 109,000 minutes of delays above Network Rail's national internal performance targets, caused by equipment failures, 63,000 affected First Great Western services alone.

First Great Western spokesman Adrian Ruck said: "The railway infrastructure on First Great Western routes is old and fragile, much of it is actually 30 years old and at the end of its useful life.

"We are working very closely with Network Rail to get track and signalling improved and we welcome Network Rail's £750m investment, which they are making over the next couple of years."

A spokesman for Oxford commuter group, OxRail, said: "This shows that more than half the country's delays are in our region.

"This comes as no surprise to local commuters. Passengers who pay vast amounts in fares are fed up with the same old excuses for delays."

The report looked at Network Rail's performance from July to September It showed that despite the summer floods, the number of trains arriving on time nationally was 88.7 per cent - ahead of the rail industry target of 88.3 per cent.

The July to September figure for First Great Western has not yet been released, but in April to June the figure were 83.8 per cent. In the same period Chiltern Railways punctuality was 94.4 per cent.

One of the country's biggest rail companies is planning to increase train fares by up to eight per cent in the New Year, a union has claimed.

Arriva CrossCountry, which runs services covering Oxford and Banbury and has taken over from Virgin Rail, will raise fares by between four per cent and five per cent above the July inflation rate of 3.8 per cent, according to the Transport Salaried Staffs Association.

A spokesman for Arriva said: "We are still finalising details of the 2008 fares structure."