Environmental campaigners are gearing themselves up for a major clash with the government over a huge new road-building programme to be unveiled by the transport secretary, Alistair Darling, tomorrow.

Two areas of outstanding natural beauty and three sites of special scientific interest are thought likely to have roads punched through them under the £2bn scheme.

The government has been swift in its attempt to quell any disquiet. A spokesman for the Department of Transport said the proposals were aimed at "reducing congestion".

The spokesman said: "People have a desire to travel more and it's not the government's role to try and stop them. We aim to try and make it as sustainable as possible."

Tony Bosworth, spokesman for Friends of the Earth, said: "If Mr Darling gives the go-ahead to £2bn of road building it will show that new Labour has conceded defeat on getting people out of their cars and abandoned any attempt at having a sustainable transport policy."

"If the government wants to solve Britain's transport crisis, Mr Darling should spend the £2bn on giving people better alternatives to using their cars."

When Labour came to power in 1997, it drastically cut back the Conservatives' road-building programme on environmental grounds. Sustainable transport has been a major theme of the party's last two election manifestos.

Shortly after being appointed, Mr Darling said: "Britain isn't big enough for us to be pouring more and more concrete over its green and pleasant land".

The new road construction programme is an amalgamation of schemes proposed by local and regional authorities.

In 1998 the government asked them to compile a "wish-list" of transport plans that would benefit their areas.

Mr Darling is expected to agree to widen the A303/A30 route to the south-west through two areas of outstanding natural beauty - the West Wiltshire Downs and the Blackdown Hills.

Campaigners claim both beauty spots could be seriously degraded by the new roads.

Another proposal is to widen 50 miles of the M6 between Birmingham and Warrington.

The £700m plan would consume thousands of acres of valuable farmland.

Mr Darling is also considering widening the M1 between Leicester and Chesterfield.

Many other less costly schemes are also under consideration.

Mr Bosworth said: "Building and widening roads as a solution to transport problems has failed in the past and there's no reason to think it will work now.

"It will just buy drivers a few years' respite before the roads fill up again, and we're back to square one."