A project to re-double a section of railway line between Gloucestershire and Wiltshire has been delayed.
Network Rail said the scheme to upgrade the route between Kemble and Swindon to a two-track operation had been put back by four months to August.
A spokesman said a high volume of engineering, and ongoing remedial work from flooding, had led to the delay.
The £45m project to re-double the track is needed before the electrification of the Great Western Main Line (GWML).
The line was "singled" in the late 1960s as a cost-saving measure by British Rail, under agreement with the government.
A spokesman said: "The very high volume of works during Easter nationally, coupled with ongoing remedial work from flooding earlier this year, means our engineering resources will now be operating at full capacity during one of the busiest phases of the Swindon to Kemble redoubling scheme.
"To minimise risk to the work, and avoid the possibility of unnecessary inconvenience to customers, we are now working towards completing the scheme between Swindon and Kemble by August 2014 rather than Easter as originally planned."
Conservative MP for the Cotswolds, Geoffrey Clifton-Brown, said: "I have been monitoring this upgrade carefully and am obviously very disappointed with this news today.
"I will be pressing First Great Western and Network Rail to make sure that completion date does not slip any further.
"I will be watching to ensure that any disruption to passengers will be kept at a minimum."
Adding the second track is expected to allow up to four trains per hour to travel on the line in each direction.