A business leader has warned of the “colossal” cost to the North’s economy if HS2 is not delivered after the Whitehall spending watchdog said it is impossible to put a final figure on the controversial project.

And Damian Waters, the CBI’s regional director for the North West, reiterated calls for the Government to get on and build the high-speed rail link, which he said could “deliver massive benefits” to the region.

His comments come after a scathing National Audit Office (NAO) report concluded that HS2 was over budget and behind schedule because its complexity and risks were under-estimated.

It also warned it was impossible to “estimate with certainty what the final cost could be” and urged the Government and HS2 Ltd to be “transparent and provide realistic assessments” of the project.

The NAO said the Department for Transport (DfT) set the original funding pot for the first stage of the line when there was only a “basic” design for the project and accused HS2 Ltd of failing to add enough contingency.

Fierce debate continues to rage around HS2, with the recently leaked Government-commissioned Oakervee Review claiming the final bill could reach £106 billion – almost double the £56bn budget allocated in 2015.

Mr Waters said the NAO report highlighted the challenges of delivering large-scale infrastructure but added: “What is clear to the CBI and business generally, is the colossal cost of not delivering HS2.

“If the Government truly believes in levelling up the regions, especially the North West, it should deliver HS2 in full.

“Doing so will allow us to unlock the Northern Powerhouse and Midlands Engine, deliver better east-west transport improvements and add rocket boosters under our economy.

“It is exactly the post-Brexit project the Government should be championing.

“HS2 is the only shovel ready project that can deliver massive benefits for the Midlands and the North and all of the UK. The CBI’s message is clear – back it, build it and benefit from it.”

Vocal supporter of HS2 and director of the Northern Powerhouse Partnership Henri Murison echoed the sentiment.

He argued that using a Benefit Costs Ratio of over £2 of benefit from every £1 spent HS2, at the current estimated cost of £88bn, remained better value for money than the Jubilee Line extension and Thameslink rail projects in London.

“We know conclusively HS2 will cost more than originally planned, but when the decision was made to go ahead, the benefits were also significantly underestimated,” he said.

Elsewhere, the chairs of all of the Local Enterprise Partnerships across the North and Midlands have written to the Prime Minister urging him to deliver HS2 without further delay.

Phase 1 of the line between London and Birmingham was due to open in 2026, but full services are now forecast to start between 2031 and 2036.

Dark clouds have been hanging over phase 2b linking Birmingham with Manchester and Leeds, amid the anger of the project’s spiralling costs.

The leaked Oakervee Review recommended a six-month pause to investigate whether the phase could be a mix of conventional and high-speed lines – a suggestion that has been met with short shrift in the North.

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.

Business leaders and politicians in the county have called for at least one HS2 stop, with Carlisle the favoured option.

Transport Secretary Grant Shapps has said the Government will make a decision on whether to go ahead with the project in “weeks rather than months”.

In response to the NAO’s findings, a DfT spokeswoman said the department “supported this review and is already acting on many of its recommendations”.

“To ensure transparency around the project, we have worked closely with the NAO to provide information on the latest cost and schedule estimates for HS2.

“We recognise that there have been significant underestimations of both the cost and schedule of HS2 in the past which is why we commissioned the Oakervee review to provide advice on whether and how to proceed with HS2.”

So far £8bn has been spent on HS2.