The Metropolitan line extension to the centre of Watford will not be delivered despite getting the extra millions London Mayor Sadiq Khan asked for, the town's Conservative MP says.
The Croxley Rail Link was given the planning go-ahead in 2013 and signed off by the then London Mayor Boris Johnson in 2015.
Last year Mr Khan said £73.4m more was needed, which the government agreed to.
The mayor is now looking at other schemes amid concerns over costs.
Transport for London (TfL) and the Department for Transport have been exploring "more affordable" potential alternatives, including a rapid bus scheme, the mayor's office has said.
Watford MP Richard Harrington said he was "very disappointed" by the London mayor and a bus route was "not good enough".
"We've given him everything he's asked for, and more. I've reluctantly come to the conclusion that Sadiq was never serious about this," he said.
The plan to extend the line as well as open two new stations was first under the control of Hertfordshire County Council.
But in 2015 TfL took over responsibility with a funding package of £284.4m.
Last year, Mr Khan told Mr Harrington TfL needed more money and the MP secured the further £73.4m. Mr Harrington said the government was also willing to hand over control of the tracks.
A spokesperson for Mr Khan said the previous mayor had committed £49m of London taxpayers' money "without working out how much it would cost".
TfL had estimated it would need "more than double the funding commitment" with a "considerable risk" the costs would rise further, the spokesperson said.
"The mayor is determined to ensure an agreement is made which guarantees value for money for London taxpayers - particularly as Londoners are being asked to subsidise a scheme outside London."
Future of funds?
The mayor's office said it had asked the government for TfL to have access to some of the profits that developers make from building near the line in the future as a solution to cover additional costs.
But a letter to Mr Khan from Communities Secretary Sajid Javid and Transport Secretary Chris Grayling reminded him they agreed the extra funding on the condition the mayor took "responsibility for meeting all future cost overruns" and its position remained the same.
The government concluded that "effectively [the mayor had] decided to abandon the scheme" and it would "contact TfL officials to discuss repayment of DfT grant".
"We do not consider it appropriate to commit to transferring government funds over to an alternative scheme in the absence of detailed work and without the involvement of local partners in discussions," the letter said.