Bedford MP Patrick Hall said a £5.5bn upgrade of a major commuter route between Bedford and Brighton must be completed in full and on schedule.
The Thameslink Programme, which aims to deliver new trains, better stations, and more frequent services on the busy route through London, is due to be completed by the end of 2015.
Mr Hall - who said passengers had endured 20 years of congestion and overcrowding on the route - said the upgrade must be delivered with "no delay, no dilution".
He said there should be no cuts to the programme despite pressure to make savings across government departments.
"The programme is integral to supporting the economy as we emerge from recession," he said.
Transport Minister Chris Mole said the Department for Transport was closely monitoring the cost of the project, but that it was committed to the pledge to provide up to 24 trains per hour through central London at peak times.