Ministers have been accused of abandoning flagship pledges to electrify major rail lines, as the troubled schemes descended into further chaos.

The upgrades of key routes to Bristol, Oxford and Berkshire have been “deferred” after spending spiralled out of control – prompting speculation they will never happen.

Separately, the rail minister refused to commit to meeting the 2023 target – already delayed by three years – to electrify the Midland Mainline which connects Sheffield to London.

A further planned electrification, across the Pennines, is also under possible threat, with no confirmation that it will go ahead until the end of 2017 at the earliest.

The announcement that the upgrades of four lines on the Great Western route have been put back indefinitely was made by rail minister Paul Maynard, in a written parliamentary statement.

The main project to electrify the rail from Cardiff to London will go-ahead, although the bill has soared from £873million to £2.8billion and there is no completion date.

But electrification between Oxford and Didcot Parkway, Bath Spa to Bristol Temple Meads, Bristol Parkway to Bristol Temple Meads and of Thames Valley lines to Henley and Windsor are on hold.

Tom Watson, Labour’s deputy leader, tweeted: “The government has just abandoned an election pledge on rail electrification. Very bad news for South West England.”

And, in the Commons, a frustrated Conservative backbencher, Geoffrey Clifton-Brown, said: “It may well be that the third runway at Heathrow is built before the Great Western mainline is fully electrified.”

But Mr Maynard insisted deferring the electrification of some routes would free up between £146m and £165m, for other improvements.

He said: “We can bring in the benefits expected by passengers – newer trains with more capacity – without requiring costly and disruptive electrification works.”

Taxpayers may face a higher bill because those new electric trains will have to be modified if they are used on the four lines that are not being electrified.

Last year, it was revealed that the Department for Transport (Dft) was faced with paying £400,000-a-day in compensation to manufacturer Hitachi, until the trains could be re-worked.

Both Network Rail and the DfT have been sharply criticised by a committee of MPs for failing to provide a schedule of when work on the Great Western line will be finished.

Meanwhile, appearing before the Commons transport committee, Mr Maynard refused to stand by last year’s timetable to electrify the Midland Mainline by 2023.

Instead, he said: “We are electrifying the line from St Pancras to Corby and Kettering to enable faster journeys for commuters on that route.

“Then we are continuing the development work as planned to ensure that we continue to improve services to Leicester, Nottingham and Sheffield, as we laid out.”