GEORGE Osborne has called on Theresa May, as she “relaunches her premiership” this autumn, to commit to building a high-speed rail link, dubbed HS3, across the north of England to unlock its economic potential.

The former chancellor said a "northern powerhouse" rail network connecting Liverpool to Hull must be planned for as the Government pressed ahead with HS2.

The final phase of HS2 is not due to be completed and running until 2033, meaning any high speed extension to Scotland is unlikely to materialise any time before 2040 or even 2050.

So far, more than 70,000 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.

It is estimated that investment in the north of England, including HS3, could create up to 850,000 jobs and generate £97 billion for the British economy.

Mr Osborne, chairman of the Northern Powerhouse Partnership, said the body wanted a government commitment to building links across northern England, starting with a line across the Pennines.

"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."

Mr Osborne, who once described Mrs May as a “dead woman walking” and who now edits the London Evening Standard, wrote in the FT: "Northern Powerhouse Rail, or HS3, must be included in the next stage of the Government's high-speed network.

"Specifically, ministers should include the planning for the future connections when they set out the design for Phase 2b of HS2 later this year, remodelling four junctions to ensure they are complimentary with the Northern Powerhouse Rail proposals, start the detailed planning work on the line itself and allocate a long-term capital budget."

Mr Osborne said plans for HS3 "will not be cheap", with some estimates for the Pennine construction reaching £7 billion.

But he added: "This new railway would transform the northern economy."

However, the former Chancellor’s comments were attacked by the RMT transport union, which branded him a hypocrite.

Mick Cash, the union’s General Secretary, 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," he added.

Mr Osborne's comments come as 50 business and civic leaders from across the north of England were due today to hand a letter to the Government demanding an increase in transport spending.

Among the signatories are Andy Koss, the Drax Power Chief Executive, Clive Memmott, Greater Manchester Chamber of Commerce Chief Executive, and Alison Birkinshaw, principal of York College.

A section of the letter 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."

In February, an independent study by the Institute of Public Policy Research North found that £1,943 was spent per head in London on current or planned transport infrastructure projects compared to an average of just £427 in northern England.

The report's author Grace Blakeley noted: "It currently takes longer to travel by train from Liverpool to Hull than from London to Paris. Building better links between the North's cities will boost the nation's economy, driving up productivity."

Last month, Chris Grayling, the Transport Secretary, angered northern politicians when he announced Government support for a £30bn Crossrail 2 scheme for London, days after axing or downgrading rail projects in Wales, the Midlands and northern England.

The Prime Minister has said she remains "absolutely committed" to delivering the North-South railway HS2 but has been cautious about supporting any HS3 link.

Phase 1 of HS2 is due to open in December 2026 and will see trains travel at high speed between London and Birmingham before running on from Birmingham on the existing West Coast Main Line.

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.