The Northern Powerhouse Partnership has hit back at suggestions “further work” is needed to justify the delivery of HS2 in the North – saying its economic impact so far has been understated.

The partnership’s director Henri Murison’s criticism came in response to a second leak of the much-anticipated Oakervee Review, which will inform the Government’s decision on whether to press ahead with the controversial high-speed rail project.

According to the review, seen by the Financial Times, the cost of delivering HS2 to rise to as much as £106 billion – more than double the project’s initial allocated budget of £56bn.

The review recommends work on phase 2b of HS2 from the West Midlands to Manchester and Leeds should be paused for six months to investigate if it could be a mix of conventional and high-speed lines.

Any HS2 link to Cumbria would fall in the third phase of work, with HS2 trains using the existing West Coast Mainline north of Wigan to the final destination in Glasgow.

And the review says “further work” is needed to assess HS2's impact on regional growth, and warned it is “hard” to say what economic benefits will result from building it.

According to the Financial Times, the review concludes that the Government should “on balance” continue with the new line but subject to “a number of qualifications”.

It has suggested savings could be made by making the private sector chip in to fund HS2 stations, lower existing specifications, improve “cost performance of the management” and reduce the frequency of HS2 trains from 18 to 14 per hour.

“Transport investment alone will not 'rebalance' the UK economy,” the review adds.

In response, Mr Murison said “we will have to wait to see if this leak is genuine” following a previous leak of the review to The Times back in November.

According to The Times, the review then said the impact of quicker journey times and improved connections between the city regions of the North and the Midlands outweighed the potential costs of HS2.

And it also warned of large ticket price rises at peak travel times without it and bemoaned the lack of alternative “shovel ready” rail improvements.

Mr Murison said: “All of the signals coming out of the official, thorough review by respected figures suggest that the only way we can truly hope to level up the UK economy and create the same opportunities for businesses and young people in the North and the Midlands as in London and the South East is in this year giving the green light to continue HS2.

“We have recommended that further analysis on regional growth benefits (needs to be done) due to the previous understatement of these.

“The North’s civic and business leaders have argued tirelessly that major infrastructure investment is so badly needed to provide the capacity so urgently needed on our rail network as well as delivering the transformational economic growth the Northern Powerhouse needs to compete on the global stage.

“This is a seminal moment in the modern history of the UK. Any failure to seize this opportunity would be a betrayal of the North, and the promise of a Northern Powerhouse.”

Greater Manchester major Andy Burnham echoed the sentiment, arguing that improving conventional lines rather than building HS2 was a “second-class option”.

“I would say this to the Prime Minister and the Government today: This is your first big test of your commitment to the North of England and we're watching very closely,” he added.

Elsewhere, the chief executives of Balfour Beatty, Skanska and Morgan Sindall are among the signatories to a letter to Prime Minister Boris Johnson urging him to back HS2.

In the letter seen by The Times, they argue it would take “many years to get an equivalent pipeline of work in place” if HS2 was cancelled.

On the flip side, a group of more than a dozen Tory MPs will reportedly meet the Prime Minister in the coming days to urge him to block HS2 and spend the money on other projects.

Mr Johnson is expected to decide within weeks whether to go ahead with construction on the first phase of what would be Europe's largest infrastructure project.

Calls for HS2 to be delivered in full have continued to be made by business leaders and politicians in Cumbria amid the ongoing war between supporters and critics and fears the line will not stretch as far as its planned ultimate destination of Glasgow.

Under current proposals there are no plans for a stop in the county – although calls continue to be made for a stop in Carlisle, while Cumbria Chamber of Commerce also wants stops at Oxenholme Lake District and Penrith stations.