The track from Kemble to Swindon is part of the Stroud Valley line
The cost of converting the railway line between Kemble and Swindon into a dual track will be about £52m, a Network Rail feasibility study has shown.
The estimate is £7m over the amount of cash already committed to the project by the government.
Conservative MP for the Cotswolds, Geoffrey Clifton-Brown, said he is concerned the shortfall will delay the scheme.
The Department for Transport has yet to comment on Mr Clifton-Brown's claims.
The MP has called for an adjournment debate in the Commons to press for an early decision on the work.
"I had [information] from Chris Mole, the transport minister, which indicates that Network Rail's latest costing is £52.4m and the committed funding is £45m, so theoretically there's a 7m gap," he said.
"I am deeply concerned that the government are not going to find that money and are going to leave a decision on this till after the election.
"If we don't get a decision to get this slotted in now, the cross rail project's going to start, the upgrading of Reading station's going to start and I think all the money and skills will be committed to those very big rail projects, so this is very serious."
Doubling the line from single-track to a two-way service will remove the bottleneck on the Cheltenham to London route.
A spokeswoman for Network Rail said: "We believe that the Swindon-Kemble project offers good value for money and our proposal has a robust business case.
"We want to maximise the benefits for passengers and we will continue to work hard on seeking full support from the government."