Under-fire British Prime Minister Boris Johnson was on Thursday accused of "betrayal" as his government scaled back rail plans in northern England, despite announcing an unprecedented ?96 billion investment for the region.

Johnson is already reeling from a raft of misconduct allegations against his MPs, and risks further damaging headlines by backtracking on a key pledge made during his successful 2019 election campaign for a new rail line between the northern hubs of Manchester and Leeds.

The prime minister had promised a "new Trans-Pennine rail route between Manchester and Leeds" as part of the Northern Powerhouse Rail (NPR) project to improve links between major northern cities.

But transport minister Grant Shapps announced on Thursday that much of the existing line would be upgraded rather than being replaced as part of the ?96 billion ($129 billion, 114 billion euros) package.

He also announced that plans to extend the new high-speed HS2 line from the East Midlands of England to Leeds would be shelved.

The money will instead be used to upgrade tracks and services between London and the Midlands, between the Midlands and Manchester and in Yorkshire.

Prime Minister Boris Johnson promised new rail infrastructure in northern England as part of his 2019 election campaign
Prime Minister Boris Johnson promised new rail infrastructure in northern England as part of his 2019 election campaign POOL via AFP / Ian Forsyth

Shapps called the plan one of the "biggest single acts of levelling up of any government in history", referring to the government aim to address economic inequality between the north of England, and London and the southeast.

It would "bring the North and Midlands closer together and fire up their economies to rival London," he added.

Matthew Fell, chief policy director at business lobby group the CBI, said the plans represented a "significant investment that will go some way towards modernising our ageing rail networks and can be delivered at pace."

"But businesses across the Midlands and Northern England will be justifiably disappointed to see the goalposts have moved at the 11th hour," he added.

Transport Secretary Grant Shapps announced a £96-billion package for rail in the region but opposition MPs said it watered down initial pledges
Transport Secretary Grant Shapps announced a £96-billion package for rail in the region but opposition MPs said it watered down initial pledges PRU via AFP / Handout

Labour transport spokesman Jim McMahon told MPs that the plans were a "betrayal of trust, betrayal of promises" and a "betrayal of investment".

"No amount of gloss, no amount of spin can be put on this," he said. "He's not just forgotten us, he's completely sold us out.

McMahon said a new line between Manchester and Leeds was promised but he added: "He's broken that promise."

Labour finance spokeswoman Rachel Reeves, an MP in Leeds, called the plans "desperately disappointing".

"What we are getting is tinkering around the edges rather than the proper transformation of transport in the north of England," she said.

"The government has announced Northern Powerhouse Rail 70 times now in the last few years and not a single spade in the ground," she added.

Minister Dominic Raab refuted the accusations, saying that the investment proved the government was "delivering action to go with the words", adding "there's never been an infrastructure project or series of projects on this scale".

Johnson's 2019 landslide was largely due to voters in traditional Labour seats in northern England switching their allegiances on a pledge to deliver Brexit and address economic disparities.

But his government is now beset by corruption claims and a border crisis with migrants crossing the Channel from France, causing a crash in his polling numbers.

The prime minister defended the plans in an article for regional newspaper the Yorkshire Post.

"HS2 will come to Sheffield, meaning a trip to or from London will take just one hour 27 minutes -- precisely the same as under the old HS2 plans," he wrote.

"Under the original blueprint, first drawn up more than a decade ago, Yorkshire would have not have seen the benefits of our investment until at least the 2040s. Levelling up can't wait that long."