Platform work to delay Paddington to Cheltenham trains
Refurbishment work at Reading station will mean trains travelling between London and Cheltenham Spa will travel at an average speed of less than 50mph.
From April the service will leave Paddington 12 minutes early but the original timetable will apply at the Gloucestershire end.
It means passengers will have to wait more than 20 minutes at Gloucester.
Network Rail apologised for the delays but said once complete, journeys would be much improved.
"The planned timetable change for April is a temporary measure owing to the works to remodel Reading station to unlock a bottleneck," a spokesman said.
'One of worst'
"Four platforms are being renovated and will be closed, with another four reopening, reducing the number of paths for trains along the route during this time.
"This means that trains from London Paddington to Cheltenham Spa have to remain for a number of minutes at Gloucester until a path is free," he added.
Barry Doe, a contributor to Rail magazine, said at the best of times the intercity service between the capital and Cheltenham was "one of the two worst" in the country.
He added the revised timetable would make the journey between the capital and Cheltenham Spa station nearly two-and-an-a-half hours.
"If you were travelling from London to Manchester instead you'd have arrived there and be walking into the city centre in two-and-a-quarter hours," he added.