An upgraded route would help traffic flow to Devon and Cornwall

Members of the South West Regional Assembly have agreed to make the A303 and A30 into dual carriageways through the Blackdown Hills, an area of outstanding natural beauty.

They rejected the option to make improvements to the A358 from Ilminster to Taunton.

The decision, agreed by one vote, now has to be approved by the government.

Species will be lost and habitats will be destroyed Gwilym Wren, English Nature

There have been plans for improving the A303/A30 corridor from the M3 to Devon and Cornwall for many years.

The proposal to improve the A303/A30 between Ilminster and Exeter as it reaches Devon and Somerset to avoid some of the current congestion bottlenecks, particularly during summer months, was the choice endorsed by Devon County Council.

But some believed widening the alternative A358 route between Ilminster and Taunton would have been more cost-effective.

Devon County Council's Executive Member for Environment and Liberal Leader, Councillor David Morrish, said: "The arguments we all put forward were very persuasive and I am delighted they were accepted by the Assembly's members.

"Now we must continue our lobbying at Government level to make sure the Secretary of State makes the right final decision.

"This route is vital for the economy of the far south west. It will provide a complete second strategic route from London to Exeter whereas the development of the A358 would not."

But Gwilym Wren of English Nature said it the plan move would be "environmentally damaging on an enormous scale".

He said: "It's going to cost about �80m more than the alternative route and it's going to be very difficult to actually build and build in the adequate protection for the people who actually live here.

"Ground water supplies will be disrupted, species will be lost and habitats will be destroyed."

English Nature says it is still hoping it can reverse the decision and believes the A358 is still the best option.