The Croxley Rail Link will not be finished until 2020, documents have revealed.

The price for the rail link has rocketed from £120 million to £284 million and this forced the government to step in and ask Transport for London to take over the scheme last year.

A TFL timetable has revealed trains will not start running until December 2019 and the project will not be completed until March 2020 at the earliest, according to Greater London Authority documents.

When the finance package for the scheme was approved by Chancellor George Osborne in this year’s budget, Mayor Dorothy Thornhill said trains would start running in 2018.

When the plans for the Croxley Rail Link were first approved in 2011, the rail link was supposed to be finished in 2016.

Despite numerous delays, London Underground has refused to back the proposed date for trains to start running in 2019, opening up the possibility that trains might not run on the new link this decade.

The report, which was presented to Transport for London’s board when the rail provider was asked to take on the scheme, added: “At this stage, and pending completion of an integrated delivery programme, LU’s view is that there is very considerable risk associated with the December 2019 target delivery date.

“This view has been made clear to Hertfordshire County Council and the DfT, and no commitment to this date has been given.”

Once the scheme is completed, it is expected there will be six Metropolitan line train services running during peak hours and four services during off-peak hours.

The Croxley Rail Link is vital to Watford Borough Council’s growth plans, with the link set to boost the Watford Health Campus scheme and the Charter Place development.

Councillor Terry Douris has rejected any suggestion that London Underground’s refusal to commit to the December 2019 was because of Hertfordshire County Council’s handling of the scheme.

He added: “The dates London Underground are using are the dates London Underground are working to.

“It is nothing to do with what has happened. I would reject any suggestion of that at all.”

David Hughes, Director of Major Programme Sponsorship, London Underground, said: “In March, following a request from the Government, we agreed to take over responsibility for the Croxley Rail Link.

“We are currently undertaking a detailed assessment of the work required to take over the service. This will then enable us to confirm the expected opening date of the extension.”

The scheme will see the Metropolitan Line routed from Croxley Station, to a new one in Ascot Road called Cassiobridge, then another new station in Vicarage Road called Watford Vicarage Road before connecting with Watford High Street Station and then terminating at Watford Junction.

Ana Vaughan, spokesman for Hertfordshire County Council said: “We are in the process of transitioning the project across to London Underground, which is going well and nothing has changed since this was first announced in April 2015.

“We understand that London Underground is still planning for work to start on the line later this year. London Underground recognises the importance of the project and has thus designated it as a separate “programme” within their delivery arm to ensure direct reporting to the LU Capital Programmes Directors.

“We expect there to be an announcement from London Underground regarding their delivery proposals for the line in the coming weeks.”