The HS2 rail scheme might not be ready before 2030 because of "dither and delay" by ministers, former Transport Secretary Lord Adonis has said.
The Labour peer said he feared the legislation needed for the project may not be passed within this parliament.
Phase one of the £33bn high-speed rail link, between London and Birmingham, is due to start operating in 2026.
The government said it was keeping to its published timetable for completing the necessary legislation.
Transport minister Theresa Villiers said suggestions that the coalition was slowing down on HS2 were "just not true".
'Little of substance'
Lord Adonis, who got the HS2 project under way while in government, said he was concerned about the pace of the scheme.
He told BBC Radio 4's Today programme: "It took the last government 14 months to set in train the whole plan for HS2 from London to Birmingham and we started the work on the route north to Manchester to Leeds.
"This government has been in office for two years and three months and very little of substance has happened since the election.
"They've carried out one consultation but we still don't have the legislation to get the line up to Birmingham, they're delaying consultation on the compensation scheme and they still haven't published the routes north of Birmingham.
"The current transport secretary announced that she wasn't even going to publish that route until the autumn.
"So it'll be nearly four years after the last election until they produce the bill needed for HS2 and I think there's now a very good chance that legislation won't pass in this parliament. It could well get caught up in all the politics leading up to the next election and we may not see HS2 now in the 2020s."
He added: "There's been endless dither and delay, no one is gripping this, no one is driving it forward. There should have been a bill for HS2 in the Queen's Speech when instead we're apparently going to be spending month after month debating House of Lords reform."
'Swiftly as possible'
HS2 was given the go-ahead by Transport Secretary Justine Greening in January, despite strong opposition from some quarters. Phase two of the high-speed link is proposed to extend the line further north to Manchester and Leeds by about 2033.
The start of a public consultation on plans to compensate people whose homes are blighted by HS2 has been delayed.
Some MPs, including many Conservatives, are worried about how the route might affect their constituents.
Last month an inquiry by the All-Party Parliamentary Group for High Speed Rail said alternatives to HS2 would not solve capacity problems on the railways.
Opponents have argued against the scheme on environment and economic grounds, and want the government to look at improving existing transport services.
Ms Villiers said: "The suggestion that we are dragging our heels on HS2 is just not true. We are keeping to our published timetable of completing the necessary legislation for phase one by the end of this parliament.
"This is a major project which will have a significant impact on the future of the country and it is imperative that we get it right.
"We will ensure it is delivered as swiftly as possible but we will not be rushed - all relevant evidence needs to be properly considered, including the views of communities potentially affected by the line, and we will be doing this within the timeframe we set out soon after the coalition entered government."