Rail campaigners have pledged to put pressure on Network Rail to deliver trains travelling from Ipswich to London in 60 minutes, after the infrastructure body admitted it had been “overly optimistic” about what it could achieve in the next five years.

A review into all of its £38billion, five-year upgrade plans across the country has been ordered by the Department for Transport after NR admitted it had over-estimated how much it could achieve.

In their General Election manifesto, the Conservatives had pledged to “improve rail connections to East Anglia, delivering Norwich in 90 minutes and Ipswich in 60 minutes”.

However, there are now fears about how long it will take the Tory Government to deliver on this promise, just a few weeks into the new Parliamentary term.

Transport secretary Patrick McLoughlin told MPs that electrification work was being “paused” on the Midland mainline and the Transpennine route between Leeds and Manchester.

But Mark Pendlington, who co-chairs the Great Eastern Rail Campaign, said the announcement did not change his intention to have the region’s required upgrades done over the course of the next decade, with “behind-the-scenes work” set to continue as before.

“We have secured commitment at all levels to the recommendations of the Great Eastern Main Line Taskforce, which will be delivered over 10 years,” he said.

In the announcement yesterday, Mr McLoughlin said he would bring about modern, faster trains to Ipswich and Norwich, through a new contract to operate the train line, which is expected to start in 2016.

MPs said they would be putting pressure on ministers to ensure the work was done.

Ipswich MP Ben Gummer, a member of the East Anglian Rail taskforce since it was set up, said it would be necessary to keep up the pressure on NR to ensure the £1billion investment in the region’s rail service was safeguarded. He said: “This region needs major investment in its rail service and that has been recognised by those at the top of the Government.

“We have to work with Network Rail to ensure that priority remains as the organisation changes.”

He said he was pleased that Mr McLoughlin had mentioned the new trains that would be required as part of the new rail franchise which is due to start next year – but said this was only one element to bring in the Norwich in 90 and Ipswich in 60 pledges.

“This is a billion-pound project. Of that £450million is track upgrades and repairs and the rest is on new trains – the new trains will be included in the new franchise, but those of us on the taskforce have to ensure the track improvements go ahead.”

John Dugmore, the chief executive of Suffolk Chamber of Commerce, warned that continued investment in rail infrastructure across the East of England was a fundamental foundation of the continued success of our local and regional economy.

“Business has campaigned long and hard to get commitments from Government for future investment. We’ll be working with our Members of Parliament to ensure any delays or cutbacks are not made here. We’ll be reiterating our message that the Government needs to back winners and investing in Suffolk and Norfolk is the cornerstone of the future success of UK plc.”

An NR spokesman said: “We have recently started a feasibility study, which will consider what infrastructure upgrades are necessary to provide better, faster rail services between London Liverpool Street and Norwich.

“In the meantime, we will continue our work to deliver a better and more reliable railway, modernising track and overhead power lines on the Great Eastern Main Line.”