Today, Roger Freeman, Minister for Public Transport, will disclose details of the final two sections of the 68-mile route between St Pancras and Folkestone which had been held over for further consultation when the rest of the route was announced in January.
Mr Freeman will tell MPs that the Government has chosen the central route through Ashford rather than a northerly route, which it had previously supported. This will be hailed as a victory by environmentalists who had been worried by the damage a more southerly approach would have caused to marshland.
Ashford Borough Council was opposed to the original route that would have gone north of the town because it meant that most trains would have avoided the town altogether and the new international station would have been connected to the main rail link only by a spur.
The compromise was made possible by the planned closure of two army barracks on the central route which now means that this part of the line can stay on the ground for all but a kilometre of tunnel, whereas previously a longer tunnel costing pounds 60m would have been required.
However, the route will require the demolition of about 16 homes and several business premises, including three listed buildings.
Mr Freeman is also expected to reveal that plans to build a tunnel under a major electricity sub-station at Pepper Hill, near Gravesend, Kent, have been scrapped in favour of a route next to the A2.
The finalisation of the route will now allow bids to be drawn up for the line. Its estimated cost has been put at about pounds 3bn and it will open in 2002 at the earliest.