The Council for National Parks (CNP) and Friends of the Lake District warned that transport minister David Jamieson's approval of the A590 High and Low Newton bypass sets a dangerous precedent for road building in nationally designated areas.
The bypass is intended to remove 90 per cent of traffic from three villages near Grange-over-Sands. Anticipated benefits, according to the scheme's supporters, would include fewer accidents, the removal of community severance and improvement of the environment.
Although the bypass goes through the national park, it was backed by South Lakeland District Council and Cumbria County Council because of safety concerns and environmental conditions in the villages. Jamieson said the government does not favour roads in areas of environmental sensitivity but insisted that this bypass is "a special case".
However, CNP head of policy Ruth Chambers accused the government of "breaking its own rules for road building in national parks", which insist that schemes should meet a national need and have no alternative. She said safety concerns could be met by enforcing the speed limit in the area and increasing rail freight capacity.
Friends of the Lake District policy officer Jack Ellerby said he is "utterly appalled" by the government's decision to support the bypass.
"The government has relegated the Lake District's landscape to the bin," he said.
Ellerby added: "Proponents of this bypass on economic grounds see it as a precursor to full duelling of the entire A590 through the Lake District. Where will the government draw the line?"