Claims that scrapping the HS2 high-speed rail project would see its £42.6bn budget redirected to conventional rail are wide of the mark.
The most likely estimate is that the Department for Transport will get only £2bn should HS2 be derailed, with as little as £670m of this going on rail, said the report from the HSR Industry Leaders Group (HSRILG).
The report also claims that up to £5bn in costs will have been needlessly lost and not recouped, while investors and employers will be put off by Britain's lack of ambition and unwillingness to invest in its creaking infrastructure.
They say it is inevitable that a similar solution to the same problems will be looked to in the future, but at a much higher cost, and cancelling the current project will cause a brain drain of engineers forced to look elsewhere for opportunities and jobs and a much needed chance to regenerate the Midlands and north of England will be lost.
Jim Steer, director of pro high-speed rail group Greengauge 21 and founding member of HSRILG, said: "HS2 is a project that will build a bright future for Britain. With the Bill for the first stage of the route now before Parliament, we felt it important to set out the hugely positive difference this project will make.
"To bring to life its contribution, we considered this question: what would be the most likely outcome should it be cancelled? Thinking this through it became clear to all of us that walking away from HS2 is a risk that Britain just cannot afford to take."
The first, London to Birmingham, phase of HS2 is due for completion in 2026 and a second Y-shaped section, taking the line to north-west and north-east England, is planned for completion around 2032/33. Including £7.5bn for the trains, the project will cost £50bn.
But the scheme has proved the most controversial infrastructure project for years, as the first stage of the project will see a high-speed line cutting through Tory heartlands in the Chilterns.
Steve Scrimshaw, the UK rail systems managing director at engineering company Siemens, said: "HS2 is about so much more than reducing journey times between London and Birmingham. It is about a once in a generation, transformational opportunity to reconnect Britain and revitalise our busy rail network.
"The advantages of this cannot be overstated and this report points to some of those. That's why we, along with other business leaders and major employers, fully support HS2 and call on Britain to push forward in developing a world-class high-speed network and take advantage of the benefits it will bring to our nation."
Anti-HS2 group, HS2 Action Alliance, said regional cities would be "milked of talent and business" if HS2 went ahead – adding that the project "would do nothing to alleviate overcrowding on local commuter trains".
The alliance's Iain Macauley added: "Several authoritative reports, produced by renowned economists and transport experts have said the same thing: London would be the main beneficiary of HS2, regional benefit would be tiny – or even negative – and as for dealing with an 'impending capacity crisis', that's complete rubbish."
He went on: "There is not, and will not be, a West Coast Main Line capacity issue based on current services.
“How on earth is a high-speed non-stopping line between Manchester, Leeds, Birmingham and London going to ease suburban London queues, or help the 100,000-plus commuters who stand every day travelling into the likes of Paddington from the west of London, which is at 99 per cent capacity at peak times, or from Surrey and Hampshire into Waterloo, which is at 91 per cent capacity?"