Part of the £2.8 billion project to electrify the Great Western rail line between London and South Wales has been "deferred", rail minister Paul Maynard said.
The project, which was due to be completed by 2018, has already been hit by delays and soaring costs.
Network Rail estimated in 2013 that the scheme would cost £874 million, and MPs on the Commons Public Accounts Committee described the subsequent budget increase as "staggering and unacceptable".
Mr Maynard said he has "deferred" the electrification of four sections of the Great Western route because newer trains with more capacity can "bring in the benefits expected by passengers" without requiring "costly and disruptive electrification works".
The four sections being delayed are:
:: Oxford to Didcot Parkway
:: Bristol Parkway to Bristol Temple Meads
:: Bath Spa to Bristol Temple Meads
:: Thames Valley branches to Henley and Windsor
No date has been given for when they will be electrified.
Mr Maynard said the deferral would free up between £146 million and £165 million, which would be used to deliver "additional benefits to passengers".
Despite the deferral announcement, Network Rail claimed "good progress is being made".
Mark Langman, the organisation's Western route managing director, said: "The Great Western main line is undergoing a huge rail investment programme to enable new and upgraded trains with more seats and faster, greener journeys.
"The changes announced today will deliver those benefits to the greatest number of passengers in the shortest possible time. The programme remains complex and challenging but good progress is being made."
But shadow transport secretary Andy McDonald urged the Government to reconsider the decision.
He said: "These electrification works have seen their cost triple and have now been shelved twice, with businesses and commuters being made to pay the price for the Government's incompetence.
"The Government have proven themselves incapable of delivering electrification works to budget or on schedule, and this is yet another example of a broken Tory promise on rail.
"It is unacceptable for ministers to renege on promises to deliver electrification of lines time and time again. The Secretary of State must reverse this decision and make clear that the Government will keep to their word and deliver the upgrades as planned."
Ministers were accused of deceiving the public when the overhaul of the Midland Mainline and the TransPennine routes were "paused" weeks after the 2015 general election.
Then-transport secretary Patrick McLoughlin went on to order the work to resume following a review by Network Rail chairman Sir Peter Hendy.