A £63m cash boost has been unveiled by the government to complete work on a station that will eventually be linked to London's new Eurostar terminal.
The station helps the Olympic transport plans
The new Thameslink stop, built beneath St Pancras station in north London, will now be fitted out with platforms and should be in use by December 2007.
Concerns had been raised the station would not be used for years due to a lack of funding.
The stop will replace King's Cross Thameslink station on Pentonville Road.
Making the announcement on Wednesday, Transport Secretary Alistair Darling, said: "This new station will provide an excellent interchange between Thameslink and national and international services.
"It will also provide better access to passengers who use the Olympic Javelin service from St Pancras to Stratford during the 2012 Games."
Passengers faced months of disruption as the work, which finished in May 2005, was carried out.
According to the Channel Tunnel Rail Link (CTRL), the "box" is large enough to accommodate a twin-platform station that will allow passengers direct access to the refurbished St Pancras International, which is scheduled to open in 2007.
Work on the station is being carried out by London Continental Railways - the company currently completing the second stage of the Channel Tunnel Rail Link from north Kent into London.
London mayor Ken Livingstone said: "We are delighted the government has stepped in and agreed to fund these works, which are vital to improving rail services in London.
"Not only will it deliver a modern, expanded and accessible station, it is a critical piece of the jigsaw in delivering our Olympic transport plans."