Ministers have been "dithering" over the the HS2 high-speed rail project, the former transport secretary who first announced the scheme said on Monday.
And the leader of action groups against the London-Birmingham scheme said it would merely "shackle the country with an extra £1,700 debt for every household".
The Government is expected to give the formal go-ahead to the £32 billion HS2 route - which passes through picturesque Tory heartlands - some time this week.
Plans were first announced nearly two years ago by Labour's transport secretary Lord Adonis. Writing in The Times on Monday, Lord Adonis said: "The only thing high speed about the development of HS2 in Britain is the treading of water."
He went on: "The valid criticism of the Government is not that it is riding roughshod over Nimbys in the Chilterns. Rather, ministers have stalled the project while dithering over straightforward local mitigation issues.
"They make the mistake of believing that opposition to big infrastructure projects diminishes the longer you drag out consultation.
"Delay simply entrenches opposition - including hostility within the Conservative Party which is dogging HS2 - and gives it the belief that 'one last heave' will see off a prevaricating government."
Saying that "no other infrastructure project matches HS2 in nationwide benefits", Lord Adonis urged Prime Minister David Cameron to "advance HS2 with all deliberate speed and with the full authority of the State".
Meanwhile, in a letter to The Daily Telegraph today, Jerry Marshall, chairman of Action Groups Against HS2, said that rather than strengthening Britain's position in the global economy, HS2 "if anything, will do quite the opposite".
He went on: "Shackling the country with an extra £1,700 of debt for every British household at a time of austerity will undermine the recovery and inevitably lead to cuts, not only in other vital rail projects, but in other public services.
"The role high-speed rail can play in our transport infrastructure is one of bringing faster, more-frequent and reliable services that benefit the whole country rather than a single, eye-wateringly expensive project that will actually increase journey times for all but the fortunate few able to afford to use it."
The Government had been due to make a decision about HS2 before Christmas, but Transport Secretary Justine Greening announced that she would make an announcement in January.
Even if all proceeds to plan, HS2's first phase, from London to Birmingham, would not be ready before 2026.
The Y-shaped extension, taking the high-speed line north of Birmingham to north east and north west England, would not be completed until about 2032/33.
Ms Greening faces opposition not only from some local councils and some local residents in areas along the route but also from some Tory MPs.
However, Mr Cameron is firmly behind the plan, which seems certain to get the go ahead, with Ms Greening announcing - possibly - plans to mitigate the worst effects of the scheme.
With the Government ruling out a third runway at Heathrow airport, HS2 has become the central plank in its transport policy.
But the project has been mired in controversy, with claims and counter-claims as to its merit.
It has also produced probably more vitriol than any infrastructure scheme for years. Indeed, the House of Commons Transport Committee, in a report on HS2, saw fit to urge both sides to cease name calling.
Those keen on the project say that it is much needed, with a Government-commissioned report by Network Rail last week supporting the pro-HS2 lobby's view that improving existing rail lines will not solve the problem of overcrowding.
Opponents of HS2 have said that the pro-lobby's calculations for the project's benefits are wildly optimistic and that the scheme is merely a "vanity project".
Emma Boon, campaign director of the TaxPayers' Alliance, said today: "High-speed rail (HSR) will leave generations of ordinary taxpayers paying for a train set that will mostly be used by the rich.
"We can't afford £32 billion for this project right now. The business case is fundamentally flawed and the Government have vastly inflated the benefits of HSR to the regional economy while hiding the true costs of the project.
"HSR is a white elephant that won't help the millions of commuters stuck on overcrowded trains up and down Britain."
The long run-up to this week's announcement has been studded with reports from various organisations pointing out, variously, the pros and cons of the scheme.
While Government, train companies and some businesses have produced figures to support an HS2 go-ahead, opponents have announced their own statistics pointing to the whole scheme being a waste of money.
One such report - from Institute of Economic Affairs in July 2011 - said there was a significant risk that HS2 would become "the latest in a long series of Government big-project disasters, with higher-than-forecast costs and lower-than-forecast benefits".
It added that the level of financial risk was "huge" and claims that the project would bridge the North-South divide and bring regeneration "should be treated with scepticism".
On the other hand, last-week's Government-commissioned Network Rail report said there was a strong business case for HS2.
The report added that HS2 would release much-needed additional passenger and freight capacity on the existing network, offer improvements in national and international connections and provide wider regenerative and economic benefits.
Sarah Lee, head of policy for the Countryside Alliance, said: "We welcomed Ms Greening's decision last year to wait before ruling on the fate of HS2," adding that the Alliance believed it "acknowledged that the environmental costs of the project had not been properly evaluated or acknowledged in the business plan, and that the route past Birmingham had yet to be properly consulted upon".
Ms Lee went on: "However the Government has not solved ether of these glaring problems, and instead looks set to give the green light to the scheme without full consideration of the devastating impact HS2 will have on Britain's countryside or the lives of the hundreds of thousands of people living along the route.
"At present the costs of HS2 - both to the environment and the public purse - are simply too great to justify going ahead with the scheme, and we therefore urge the Government to reconsider the impact that HSR will have on Britain's countryside and rural communities before taking this decision."