Fast railway to link London and Birmingham

The announcement will reveal whether Adonis has opted to include a direct link to Heathrow airport, favoured by the airport's operator BAA, owned by Spanish group Ferrovial.

Recent media reports have suggested Adonis is unwilling to extend the route to Heathrow to the west of the capital and will favour an indirect rail link instead.

The line is expected to cut through unspoilt countryside designated by the government as an "outstanding area of natural beauty," risking objections from environmental groups in the run up to an election due within weeks.

Local communities along the route linking the two cities, 100 miles apart, are also likely to raise concerns.

Consultation on the route will take place later this year, although construction will not start for many years and will require billions of pounds of state investment.

The proposals will also include options for extending the route to the north of England and Scotland.

However, the plans may be torn up if the Conservative party wins the election expected on May 6.

The Conservatives, ahead in opinion polls, support the expansion of high speed rail but have turned down the chance for an early sight of the plans, saying they did not want to be tied to the government's route.

They want a high speed link to Heathrow and say their line would run through Birmingham to Manchester and Leeds in the north.

The government will publish maps of its preferred route accurate to five metres in urban areas and to 25 metres in rural districts, a Department for Transport spokesman said.

Britain only has 68 miles of high-speed rail, linking London to the Channel Tunnel subsea railway to France.

A 200 mph route would cut journey times to the north of Britain by half to just over two hours.

(Reporting by Tim Castle; editing by Elaine Hardcastle )