The CPRE says 21 road schemes have doubled in cost

Campaigners say that of 151 road schemes recently completed or which are still on-going, 96 have gone over budget and 21 have doubled in cost.

They say the bill for 39 national road schemes has risen by an extra �1.3bn.

The Highways Agency said it strived to reduce expense, but land and property value fluctuations affected costs.

The Campaign to Protect Rural England (CPRE) says costs often "escalate" and the practice "needs to stop".

Campaigners, who refer to figures released in answers to Commons questions by MPs Colin Challen and John Hayes, say building just one mile of motorway costs on average �23m and a mile of dual carriageway �12m.

'Sustainable' transport

The figures have been highlighted on the eve of the deadline for local authorities to submit their five-year local transport plans to government.

These plans are set to include proposals for more roads.

Campaigners said more money should be invested in "sustainable" transport measures, such as public transport, safety schemes and improvements for walking and cycling.

CPRE's head of transport policy, Paul Hamblin, said: "The Transport White Paper recognises that road building offers no long-term solution to today's transport problems.

"And yet ministers and many local authorities continue with plans which will see more tarmac laid over green fields.

Reducing costs

"These figures show that there is a high price to pay for the taxpayer as well as everyone who cares for our countryside."

However, the Highways Agency said budgets had been better controlled in recent years - partly because projects were being designed more carefully in their early stages.

It said it was doing all it could to keep costs down, attributing rising expense to other factors.

A Highways Agency spokesman said: "There are good reasons why estimated costs have increased on road schemes.

"Preparation and preliminary costs are now more clearly identified, rather than contained within construction costs.

"There is a tendency for more design work to be carried out earlier, to identify and manage project risks, which should lead to reduced out-turn costs."