The mayor of London has vowed to keep the Metropolitan Line Extension project on track – if more money is coughed up.

Sadiq Khan has responded to a letter sent by Mayor of Watford Dorothy Thornhill and leader of Hertfordshire County Council in February, which slammed his decision to scrap the scheme.

He has written back to them this week reassuring them he shares their “disappointment and frustration” the scheme has reached an impasse.

In his letter – which has been seen by the Observer - he blamed the project tripling in cost and being several years behind schedule.

The letter read: “While the Government did offer to close the funding gap on the project, this was conditional on TfL accepting any future cost risk, and it failed to address the real possibility of substantial further cost increases.

“I firmly agree with you that the MLX has the potential to contribute both to better transport connections and to sustainable growth and addressing the housing challenge in Watford and wider region.

“TfL has not abandoned the scheme. If an alternative funding package could be agreed TfL remains willing to deliver the scheme.”

Jagtar Singh Dhindsa, Watford Labour's mayoral candidate, says the letter shows Mr Khan has not abandoned the MLX and wants it to go ahead if the funding issue can be resolved.

He said: "We have been in regular touch with the London Mayor's office and obtained assurances that the MLX scheme would be protected while a way out of the impasse is negotiated.”

However, Watford’s leading politicians say the London Mayor is being “completely disingenuous” by claiming he is supporting the scheme.

MP for Watford Richard Harrington said: “I am hugely disappointed in the London Mayor Sadiq Khan. The Met Line Extension is only five miles of track, and in June last year he wanted an extra £50 million, and by October it was £73 million.

“Even so, I have jumped through every hoop to get this project up and running, and each time he blocks it. The Met Line Extension has the support of two Secretaries of State, with nearly £200 million of government funding.

“Let's not forget that TfL have also been promised the development rights - worth up to £45 million - and will also profit from the ticket sales. His budget already includes a £46 million contingency, this is not about money. He just doesn't want to do it, and that's clear now."

Mayor Thornhill argued that Mr Khan has never been willing to come up with an alternative plan.

She said: "This is just a case of the Mayor of London and TfL moving the goalposts on Watford residents yet again. As noted in his letter, the government has met the funding gap identified by TfL. But now we learn that despite this being a TfL scheme, the Mayor also wants them someone else to cover the financial risk involved in the project too. This would be like building an extension, then asking your next door neighbour to foot the bill if the hole thing collapsed tomorrow.

"Although I welcome the Mayor's comments that they have not 'abandoned' the scheme', it is simply empty words unless he is prepared to help fund it.

"I call on local Labour party councillors to put Watford residents first and stand up to their mayor, rather than running scared of the impact this might have on them in the ballot box on May 3."

Hertfordshire County Council has been contacted for a comment.