Council would be responsible for any overspend on £104m North West Relief Road

Any extra cost would be on top of the £21m – according to current estimates – that the authority would have to contribute if the plans get the green light.

Shropshire Council said it is on target to submit an outline business case for the project to the Department for Transport (DfT) by December 22.

The authority will be asking the Government to fund about 80 per cent of the cost from a fund set up for large local transport schemes.

But Shrewsbury Friends of the Earth has said that Shropshire Council should put the relief road scheme "back in the drawer".

Dave Green, of Shrewsbury Friends of the Earth, said: "It is simply not credible that Shropshire Council could take on such an open ended commitment in these times of financial constraint.

"It is time that Shropshire Council put the scheme back in the drawer and concentrated on more achievable and less controversial transport improvements."

Frank Oldaker, also from Shrewsbury Friends of the Earth, added: "This alone is unacceptable when the council are cutting services and say they are facing a £36million shortfall in their overall budget for next year - but what is really worrying is that they will also have to find the money to pay for any increase in the cost of the scheme.

"As everyone knows the cost of major civil engineering projects often increases dramatically and the Government are quite clear that no further money will come from them."

Steve Davenport, Shropshire Council’s Cabinet member for highways and transport, said that it is "standard practice" for local authorities to foot the bill for any overspend and that the road would be a good investment.

He said: “Our current estimate of the cost of building a North West Relief Road (excluding Oxon Link Road) is £104 million.

"Shropshire Council will be asking the Government to fund about 80 per cent of this (about £83 million) from a fund specially set up for large local transport schemes. The council expects to make a ‘local contribution’ of about 20 per cent of the total cost (about £21 million).

“Shropshire Council is preparing an outline business case for consideration by the Department for Transport. All cost estimates are currently being reviewed and updated for the outline business case.

“Based on earlier studies, the economic benefit of building the road – ie the discounted sum of all the forecast journey time, fuel and accident savings over a 60 year period - is expected to exceed the cost. And if the business case is deemed to be sound by the Department for Transport this would suggest that a commitment to spend about 20 per cent of the total cost could be considered a good investment for Shropshire Council.

“It’s important to note that taking local responsibility for any overspend is standard practice on major schemes of this type. The full business case that would be prepared in due course will acknowledge areas where there is the potential for such, and will accommodate these in the final funding grant bid in order to manage this.

"At this time there is also no prescribed level of local funding stipulated by DfT, and this amount will be confirmed in due course.”