It confirms that most possible schemes to avoid damage to the environment by putting the line through tunnels have been abandoned to keep costs down.
Protesters are likely to have only another six months' consultation to try to change the route before it is published in a parliamentary Bill.
John Prescott, Labour's transport spokesman, condemned the plans as the 'cheapest option you could get'. He accused the Government of reneging on promises to protect the environment with tunnels which would have cost an additional pounds 800m.
The leak of a two-page supplement prepared for a Kent newspaper by Union Railway, the BR subsidiary promoting the link, upstaged the announcement due to be made next week by John MacGregor, Secretary of State for Transport.
Mr MacGregor was prevented from making his statement on Wednesday this week by Labour, who protested that it would overshadow Labour's attack on the Budget.
The leak confirmed that the route through Kent will run from the tunnel entrance near Folkestone through Ashford, Charing and Northfleet station, before crossing the Thames in a tunnel at the M25 Dartford crossing.
No details were given of the route into St Pancras, but sources said it was wrong to assume that the London section had not been agreed. As revealed last year in the Independent, the Government favours a cheaper route, using St Pancras as the terminus rather than a new low-level station at King's Cross. The St Pancras route would run alongside the North London line and through a new tunnel at Dalston.
This was only proposed late last year by Union Railway, and a final decision awaits an engineering survey expected in October. The original route would have involved a tunnel from Stratford to King's Cross.
The documents leaked yesterday show the planned route in Kent would cross the Boxley Valley, a beauty spot near Detling, and run within 300 metres of houses on the Bluebells Estate.
Sandy Bruce-Lockhart, Kent County Council's planning committee chairman, said: 'It is extremely unfortunate the news of the route has come to the people of Kent via a leak. And we will now be looking for the Government to give full details as quickly as possible.'
Difficulties over funding the rail link still threaten to delay the scheme. The Government accepts it will have to be a joint public-private sector project, rather than entirely funded by the private sector.
In his Budget speech Norman Lamont said it will open 'around the end of the decade'. However, the difficulties in developing such joint sector projects were illustrated by the lengthy dispute over the Heathrow Express rail link and questions will remain over Mr Lamont's target date until the Treasury shows it can relax its rules and make it easier for such projects to proceed.
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