Former Chancellor George Osborne has urged the government to build high-speed rail lines across the north of England, from Liverpool to Hull.
Mr Osborne, who launched the “Northern Powerhouse” initiative when in government, called for the commitment in an article in the Financial Times.
He admitted “it will not be cheap”, but said it would “transform” the economy.
The government said it was “investing billions of pounds” to “better connect communities” across the north.
The Northern Powerhouse Partnership, which is chaired by former Tatton MP Mr Osborne, is launching a campaign for the new-high speed connection, starting with a line across the Pennines.
Plans for “HS3” would follow on from the existing HS2 scheme – a planned line linking London and Birmingham that will split into two branches to Manchester and Leeds.
The partnership wants the government to redesign the second phase of HS2 to “remodel” four junctions, which could then be used for further connections under their rail proposals.
Writing in the FT, Mr Osborne said the new railway would “bring seven million extra people – and three times the number of businesses – within a 90-minute journey time of one of the northern cities”.
He said the estimated cost of the Pennines line had been put as high as £7bn, but argued the investment could be spread over many years and the transport budget was built to take in such large projects.
“There is no geographical reason why this cannot happen,” wrote Mr Osborne.
“The distance between Manchester and Leeds is shorter than the length of the Central line on the London Underground.”
‘Exists and breathes’
He said there had been a “systematic attempt” to “eradicate all mention of the initiative” by some of Theresa May’s advisers.
But he told BBC Radio 4’s Today programme the idea was still “thriving”.
“Now the idea is not just dependant on the political career of one chancellor or one prime minister, it exists and it breathes and it lives in the north of England.”
Andy Burnham, Labour’s Mayor of Greater Manchester, tweeted a link to Mr Osborne’s article, adding that the north of England “is getting organised”.
Last year, the prime minister vowed to press ahead with the project.
Writing in the Yorkshire Post, Mrs May promised to “help the great cities and towns of the North pool their strengths and take on the world”.
However, last month the government scrapped the planned electrification of railway lines in Wales, the Midlands and the north of England, prompting anger from local authorities and businesses.
Days later, Transport Secretary Chris Grayling backed proposals for Crossrail 2 – a north-east to south-west railway in London.
A spokesman for the Department for Transport said the government had already made a commitment to Northern Powerhouse Rail, giving £60m to Transport for the North to develop plans, adding: “[We] look forward to working with them once proposals are submitted later this year.”
“We are also investing billions of pounds across the north of England to better connect communities, build the Northern Powerhouse, and deliver improved journeys right across the region,” the spokesman said.