The total economic benefits of HS2 will be significantly diminished if the Northern legs of the high speed rail line are delayed or cancelled.

That’s the message from the Association for Consultancy and Engineering (ACE) and its 400-plus members, for whom the lingering doubts over the future of HS2 have raised serious concerns.

In an open letter to the Government published on its website, ACE is calling on Government to collaborate with industry leaders as a matter of urgency, as the potential cancellation of HS2’s future phases exposes its inconsistent approach to infrastructure.

It is asking the Government to honour its promises to level up the North through this infrastructure project, that will shape the future of the UK.

The letter from ACE's 400-plus members states: "The high-speed railway project has been a focal point of the UK’s infrastructure ambitions for several years and has been inextricably linked to the levelling up agenda, by improving connectivity between the capital and our Northern cities.

"HS2 was meant to be transformative for the UK, driving economic growth, job creation and regional development.

"But the uncomfortable uncertainty that now surrounds it, seriously threatens the economic benefits it was intended to bring."

ACE members also note that with media speculation rife that the project could be “cut short”, effectively cutting off the Northern cities that were set to benefit from a much-improved rail link to London, the "promises Government has previously made on regional development are now in question".

In addition, members say other projects that would have come off the back of it, from housing to commercial ventures, are also now in jeopardy.

ACE members point to the fact the Eastern arm of the project, which would have taken passengers to Nottingham and Leeds, has already been scrapped.

But now, with the link to Manchester as well as the last five minutes into the centre of London under threat, ACE members are appealing to the Government for stability, investment and collaboration with the private sector.

"Doing away with HS2’s Northern lines will create significant costs – both social and financial," the members of ACE say.

"Investors are deterred when projects are frequently redesigned or altered – and the overall cost of the project spirals.

"When you modify or redesign an infrastructure project of this scale, the cost increases in the long term, as each change requires additional planning, engineering and procurement."

ACE has previously highlighted how alterations to the scope of HS2 also risk damaging the UK’s reputation for delivering significant, high quality infrastructure projects.

In its letter to the Government, ACE members say they have demonstrated time and time again that they have impressive skills and expertise – and lead the world when it comes to project delivery on this scale.

However, the state of the HS2 project, which has faced delay after delay, "is painting an unfair and unjustified picture when it comes to our nation’s ability to deliver on big infrastructure projects, unjustly eroding our credibility," they say.

"We urge the Government to collaborate with industry now to ensure the efficient delivery of this vital infrastructure project and keep the UK’s infrastructure ambitions on track, delivering prosperity to every corner of the nation," says the ACE.