Raising the level of a Fenland road in order to cut the time it is shut by floods could cost almost five times as much as previously thought.
The warning about the A1101 Welney Wash Road was delivered during a floods summit held today.
But the area’s MPs have urged officials to continue working on the project.
Norfolk County Council engineers have been carrying out a feasibility study on ways to improve the road, which is currently closed because of flooding.
The meeting in Downham Market’s town hall was told that closures caused by flooding would be reduced by more than 50 per cent if the minimum height of the road was raised by one metre.
Initial estimates suggested the work would cost around £1 million to complete.
But Dave Stephens, the authority’s network management team manager, said he feared achieving the full metre height increase would cost around £5 million, because of the need for more extensive works.
Lower cost alternatives are also being examined, including raising the road by half a metre, which Mr Stephens said could be delivered for around £1.1 million.
But he admitted the council could not afford to fund the scheme alone, adding: “It would need a significant contribution from other sources.”
Environment Agency officials said money was available in its current investment programme to go towards the project.
Environment secretary and South West Norfolk MP Elizabeth Truss, who chaired the meeting, also indicated that talks should be held with Cambridgeshire County Council and the area’s two local enterprise partnerships to encourage them to contribute.
But she added: “I’m keen to see the project progress as soon as possible. It does seem to be taking quite some time.
“It’s an A road. It’s not acceptable for it to be closed for so long. I think the costs of that are growing for local residents.”
The meeting was also told of concerns about the signs warning drivers about the depth of flood water on the road not recording the maximum depth.
A driver had to be rescued by fire crews yesterday after becoming stranded in the waters.