Fresh doubt is being raised over the estimated cost of a proposed northern bypass in Ipswich following the publication of an outlne business case.
Figures unveiled on Tuesday suggested that an inner route - one connecting the A12 at Martlesham with the A14 just south of Claydon - could be delivered for less than £400million if building work began today.
READ MORE: Two Suffolk authorities withdraw northern bypass support
That figure rises to £500-560m in 2027, which is the earliest it could be constructed.
But sources indicate that those numbers will not be enough, and the road could not realistically be delivered for less than £800m-£1bn.
Andrew Stringer from the Green group at Suffolk County and Mid Suffolk councils had been critical of cost estimates of the Upper Orwell Crossings, which ended up ballooning by around £40m and causing the project to be axed, and voiced fears once again.
He said: "Following trends of past estimates for some of this work, we need to take everything with a huge pinch of salt.
"The most economically literate people will not believe we can deliver this road for anything like this price."
It is understood that transport chiefs in Whitehall have also raised serious doubts about the £500m estimate.
Consultancy firm WSP put together the numbers in the northern bypass strategic outline business case, and was also the firm responsible for figures in both the Upper Orwell Crossings in Ipswich and the Sudbury bypass scheme.
That Sudbury scheme had to be scrapped because highways chiefs felt the £50-70m cost would not be funded by the Department for Transport - a figure which escalated from £40m in 2017.
Despite the failings around those two schemes, the firm said its figures were correct.
WSP project director Graham Higgins, said: "WSP is confident in the estimated scheme costs presented in the Strategic Outline Business Case for Ipswich Northern Route. Please refer to Appendix E for details of how these costs have been estimated."
Suffolk County Council leader Matthew Hicks said: "They are experts in their field.
"It is one of the largest organisations in the industry who we would do a business case with."
The report has received a mixed response, with three authorities withdrawing support for the scheme since Tuesday morning - East Suffolk, Mid Suffolk and Babergh.
However, Ipswich MP Tom Hunt and Ipswich Borough Council leader David Ellesmere have united in calls to press on with the project.
Suffolk County Council's cabinet will make a decision on whether to continue work at its meeting on February 25.
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