Multi-billion pound proposals to widen the M25 and improve other motorways will threaten the proposed South Downs National Park, campaigners say.

Transport secretary Alistair Darling was outlining huge road-building programmes today, which includes introducing road charging in England.

The proposals have been welcomed by drivers but have horrified green groups.

Environmentalists fear the motorway widening projects will cut across beauty spots, while the haulage industry hails the plans as a chance to improve the country's road infrastructure.

The Campaign to Protect Rural England said: "The roads announcement threatens large areas of countryside, including the proposed South Downs National Park."

He added the expected decision to widen much of the M25 to four lanes in each direction would merely mean "more Tarmac for more traffic".

Geoff Dossetter, of the Freight Transport Association, said: "The UK deserves a roads infrastructure appropriate for the fourth largest economy in the world."

As well as the decision on the M25, Mr Darling was expected to announce widening schemes for a number of other motorways, including the M1, the M18 and the M62.

His decisions will be based on recommendations from transport studies which have examined road-building and other transport improvements.