Former chancellor George Osborne has called on the Prime Minister to commit to building a high-speed rail link from Liverpool to Hull to unlock the north of England's economic potential.
The ex-Tory MP said a "northern powerhouse" rail network connecting the west and east coasts must be planned for as the Government presses ahead with HS2.
So far, 80,000 people have signed a petition demanding more investment in transport outside of London and the South East.
Earlier this year it emerged that more than half of England's annual £32.7 billion transport budget is spent in London, while an independent study by think tank the Institute of Public Policy Research (IPPR) North found that £1,943 is spent per head in London on current or planned transport infrastructure projects, compared to an average of just £427 in the north.
It is estimated that investment in the north, including the HS3 rail-link, could create up to 850,000 jobs and generate £97 billion for the economy.
Mr Osborne, chairman of the Northern Powerhouse Partnership, said the body wanted a Government commitment to build links across the North, starting with a line across the Pennines.
Writing in The Financial Times, Mr Osborne said: "The Northern Powerhouse Rail fits with Mrs May's stated objective of building an economy that works for everyone.
"Far be it from me to offer advice to the Prime Minister on how to relaunch her premiership this autumn, but making this big commitment to the North at the Conservative conference in Manchester would not be a bad place to start."
The 46-year-old conceded that HS3 would "not be cheap", with some estimates for the Pennine construction reaching £7 billion, but added that "this new railway would really transform the northern economy".
Andy Koss, Chief Executive of Drax Power, a biomass and coal-powered station near Selby, said it can currently take trains up to 10 hours to transport biomass the 90 miles from Liverpool to the North Yorkshire power station.
He estimated that HS3 could cut the journey times by between four and seven hours.
While Liverpool metro mayor Steve Rotheram said the case for Northern Powerhouse Rail is focused on the need for "a balanced, productive and resilient" UK economy, he continued: "Of course, one cannot pass over the fact that George Osborne could have made a practical rather than merely rhetorical contribution to rebalancing the UK economy during his lengthy tenure as chancellor of the exchequer."
However, Rail, Maritime and Transport (RMT) union General Secretary Mick Cash accused Mr Osborne of "hypocrisy" over his comments.
He said: "This is a man who was the key player in governments which presided over fragmented, cash-starved and privatised rail across the North and which put profiteering first while passengers were left rammed into clapped-out, lashed-up Pacer trains.
"The real legacy of George Osborne's period in government is axed electrification, modernisation and renewal programmes, and private train companies given a political instruction to axe safety-critical guards from their trains."
While Theresa May has said she remains "absolutely committed" to delivering HS2, the north-south high-speed rail link, she has been cautiouos in supporting HS3.
Mr Osborne's comments came as 50 business and civic leaders from across the north of England handed a letter to the Government demanding an increase in transport spending.
The letter, published in the Yorkshire Post, reads: "Connecting our great cities of the North with a world-class, higher-capacity rail network is not only fundamental to the success of the Northern Powerhouse, it is fundamental to the success of the entire country.
"We are calling on you to back this success and back NPR."
The latest calls to the Government come just a month after Transport Secretary Chris Grayling angered politicians in the north by announcing support for a £30 billion Crossrail 2 scheme for London, just days after axing or downgrading rail projects in Wales, the midlands and the north.
Tens of thousands of commuters have signed up to the IPPR petition calling for the promised Manchester-Leeds trans-Pennine electrification to go ahead.
Some 80,000 people have signed an IPPR petition calling for the promised Manchester to Leeds trans-Penine electrification to go ahead.
The petition also calls for £59 billion of investment in northern transport to match money spent in the south.
On Wednesday, a major cross-party summit will be held in Leeds to discuss transport investment in the north.
Plans for HS2 are underway, with phase 1 - which will see trains travel at high speed between London and Birmingham before running on from Birmingham on the existing West Coast Main Line - due to open in December 2026
A second Y-shaped phase will open in two stages.
Phase 2a from the West Midlands to Crewe will launch in 2027 and phase 2b, from Crewe to Manchester and from the West Midlands to Leeds, South Yorkshire and the East Midlands, will open in 2033.