The 14 miles of road south of Newport will cross the Gwent levels, marshy pasture criss-crossed by drainage channels rich in rare plants and insects. Much of them are government-designated Sites of Special Scientific Interest. The levels have been farmed for thousands of years and have archaeological remains.
Mr Redwood said that the six-lane motorway, costing pounds 320m, was needed because 'very soon the increasing delays caused by congestion will become unacceptable'.
However, Robin Pellew, director of the World Wide Fund for Nature, said: 'With one breath the Government espouses the idea of sustainable development, then with the next it proposes to blitz a mega-highway through some of the most important conservation areas in Wales.'
The new road, which may be tolled, will meet anticipated traffic growth.
Environmental groups said Mr Redwood was stuck in a time-warp, lagging behind the Department of Transport which had begun to recognise the impossibility of meeting rising demand for travel by providing ever more road space.
The Secretary of State's statement boosted plans by Greenpeace, Friends of the Earth, WWF, Transport 2000 and other green organisations for the first national anti- roads rally, which they hope to stage in Trafalgar Square, London, in October.