Phase one of the HS2 high-speed railway between London and Birmingham faces a delay of up to five years, Transport Minister Grant Shapps has said.
The first phase was due to open at the end of 2026, but it could now be between 2028 and 2031 before the first trains run on the route.
Mr Shapps said HS2’s cost had risen from £62bn to between £81bn and £88bn.
The second phase to Manchester and Leeds was due to open in 2032-33, but that has been pushed back to 2035-2040.
Mr Shapps’ statement was based on a report from the chairman of HS2, Allan Cook, which concluded that the new railway could not be delivered within the current budget.
“I want the House to have the full picture. There is no future in obscuring the true costs of a large infrastructure project – as well as the potential benefits,” said Mr Shapps.
Mr Cook’s report comes ahead of a government decision on whether HS2 will go ahead at all. Last month, the the government said it planned to review the costs and benefits of the rail project, with a “go or no-go” decision by the end of the year.
Originally expected to cost £56bn in 2015 prices, Mr Cook said the new cost estimate was adjusted for inflation, and based on today’s prices.
Mr Cook, who started his role in December, had already warned about the overspend while preparing a review of the project’s cost and schedule.
He told the Department for Transport last month that the scheme could not be delivered within its budget.
“The budget and target schedule for the programme have proved unrealistic, while at the same time the benefits have been understated,” Mr Cook said.
Concerns that rising costs and delays could threaten the viability of HS2 are not new. Documents seen by the BBC last month, showed that both the government and HS2 knew the new high speed railway was over budget and probably behind schedule years ago.
In July, Meg Hillier, chairwoman of the Public Accounts Committee, cast doubts on the 2026 opening target, calling it “unrealistic”.
What is HS2?
HS2 is a new railway line which, once completed, would run from London to the West Midlands, Manchester and Leeds.
Trains on the London to Birmingham route would be 400m-long (1,300ft) with up to 1,100 seats and would be capable of reaching speeds of up to 250mph. They would run as many as 14 times per hour in each direction.
The Department for Transport says the project will cut Birmingham to London journey times from one hour 21 minutes to 52 minutes.
Once the second phase is complete, Manchester to London journeys would take one hour seven minutes (down from two hours seven minutes), and Birmingham to Leeds 49 minutes (down from two hours).
This would effectively reduce journey times between London and Edinburgh and Glasgow by an hour to three-and-a-half hours.
The government hopes its creation will free up capacity on overcrowded commuter routes.