London Underground has confirmed that plans are in place to start construction work on the extension of the Metropolitan line to Watford in 2016.
They aim to complete the new link in 2020.
Two new stations will be built at Cassiobridge and Watford Vicarage Road, connecting West London better to Watford, and to the West Coast Mainline via the station at Watford Junction.
A full funding package for the extension has now been agreed between the Department for Transport (DfT), Hertfordshire County Council and Transport for London (TfL), and the work will be delivered by LU. The existing Watford Metropolitan station will close to the public following the opening of the new stations, but will be retained as sidings for the extended railway.