HS2 might not be finished until 2040 and could cost as much as £78 billion, the firm behind the project has admitted.
In a statement to Parliament, Transport Secretary Grant Shapps said that talks had been held with the chairman of HS2, who had doubts about the current completion date and budget.
He said: "Colleagues will see that the chairman of HS2 does not believe that the current scheme design can be delivered within the budget of £55.7 billion, set in 2015 prices."
The first stage of the line, running from London to Birmingham was initially scheduled to be finished by 2026. This has now been revised to as late as 2031.
Phase 2b, running from Birmingham to Leeds and Manchester is now expected to take as long as 2040 to complete, seven years later than the previous delivery date of 2033.
The firm says the entire project could cost as much as £78 billion in 2015 prices (or up to £88billion in 2019 prices) - up from the previous budget of £55.7 billion set in 2015.
Today's remarks from the HS2 firm come just weeks after the Prime Minister ordered an independent review into "whether and how" to proceed with the project.
Boris Johnson has previously said he had doubts over the business case for HS2 and claimed the money could be better spent on high-speed rail links in the north of England.
Critics have argued that the line is essential for regions including the West Midlands whose economic growth is dependent on high-speed rail connectivity.
The review is due to be published before the end of the year.
Watch ITV Central's special programme on HS2 below: