Rail commuters fed up with delays and cancellations are being urged to bombard Transport Secretary Ruth Kelly with letters in support of a upgrade scheme.

The appeal comes from the county council, which says it is "taking nothing for granted" over plans to reinstate double track on 20 miles of the Cotswold Line, at an estimated cost of at least £51m.

Network Rail is seeking consent to spend the money from the Office for Rail Regulation and is expecting a decision early next month.

The project to increase capacity and cut delays would include an extra set of rails running for four miles, from just east of Charlbury to Ascott-under-Wychwood.

The overall effect of this, and more double track in Gloucestershire and Worcestershire, according to Oxfordshire County Council's rail development officer Adrian Saunders, would benefit commuters into and out of Oxford and beyond.

He said: "A lot of performance modelling has gone into improving the performance on this line and the lack of double track is the biggest single constraint in improving the service.

"Our policy is to encourage people to get out of their cars and use public transport.

"With more trains, and more people using them, delays are a daily occurrence for commuters, as trains wait to get on the single track between Wolvercote and Ascott-under-Wychwood.

"With pressure on Government finances, the council knows that nothing can be taken for granted."

The council is writing to Ms Kelly and urging commuters to do the same.

Much of the Cotswold Line was reduced to a single track by British Rail in 1971. Passenger train operator First Great Western says it is the worst-performing route on its network and blames the long single-track sections.

Ian Hudspeth, the council's cabinet member for transport, said: "We have to make sure the Government knows how important redoubling is for everyone who travels by train along the Cotswold Line, and from Oxford and Didcot, as they also get caught up in the delays.

"I urge everyone who travels by train to get to and from work to lend their support, by writing to the Secretary of State."

People should write to: Rt Hon Ruth Kelly MP, Secretary of State for Transport, Department for Transport, Great Minster House, 76 Marsham Street, London, SW1P 4DR.