The government is launching a review of high-speed rail link HS2 - with a “go or no-go" decision to be made by the end of the year, Transport Secretary Grant Shapps has said.
The review will consider whether and how the project to connect London, the Midlands and northern England should proceed, looking at costs and benefits.
Mr Shapps refused to rule out scrapping it entirely.
He said it was "responsible" to see whether HS2 was "going to stack up".
Phase 1 of the development between London and Birmingham is due to open at the end of 2026, with the second phase to Leeds and Manchester scheduled for completion by 2032-33.
It is designed to carry trains capable of travelling at 250mph.
When asked about the billions already spent on the project, Mr Shapps said: "Just because you've spent a lot of money on something does not mean you should plough more and more money into it."
He said ministers were asking the reviewers "just give us the facts."
"Go and find out all the information that's out there… genuinely what it would cost to complete this project, and then we'll be in a much better position to make that decision - go or no-go by the end of the year."
The review will be chaired by Douglas Oakervee, a civil engineer who served as chairman of the Crossrail project between 2005 and 2009.
Lord Berkeley, another civil engineer who worked on the construction of the Channel Tunnel, will act as his deputy.
A final report will be sent to the government in the autumn.
During the Conservative Party leadership campaign Boris Johnson said he would not scrap plans for the new rail link, but did express "anxieties about the business case".
Previous governments have argued the new route would boost the economy, but concerns have been raised over the cost and route.
In July, the current chairman of the project reportedly warned that the total cost could rise by £30bn - up from the current budget of £56bn.
Labour peer Lord Adonis, a former transport secretary who worked as an infrastructure adviser to Theresa May, said the review was "as stupid as you can get" and would "screw Birmingham and the North".
He tweeted that it would become "a massive bun fight, while the transport department runs for cover and HS2 Ltd is paralysed by indecision".