The controversial Norwich Western Link will now cost £100m per mile, according to campaigners who say the scheme should be scrapped.
But Norfolk County Council, which has yet to update its cost estimate for the 3.9-mile road since last summer, said it "did not recognise" that figure.
In June last year, the council said the price tag had gone up £50m to £251m, from the 2021 figure of £198m.
But Labour councillors against the scheme calculated the road, which would connect the Northern Distributor Road to the A47 west of Norwich, will now cost £400m, once borrowing and interest rates are taken into account.
The Conservative-controlled council recently voted to wind down work over a planning application for the scheme amid uncertainty over the funding for the scheme.
READ MORE: Norwich Western Link route removed from Ordnance Survey maps
The government has yet to announce whether it will back the business case for the scheme and bankroll 85pc of the cost.
If it does, then the council would still have to use reserves or borrow to cover the rest of the bill.
Andrew Jamieson, the Conservative-controlled council's cabinet member for finance confirmed at a meeting of the council's corporate select committee on Monday (July 10) that borrowing money would now cost the council £100,000 for each £1m borrowed.
Following that meeting, Labour said the council would have to borrow at least £38m over 50 years and, at around 5pc interest, that would come to £190m in interest and repayments, including the principal loan.
The party said that would push the total bill, on top of the £213m cost, minus the £38m loan, to £403m.
READ MORE: £40m spent on Norfolk's 'doomed' Norwich Western Link road
Emma Corlett, deputy leader of the Labour group, said: "It beggars belief that county hall Conservatives still let this stagger on despite the spiralling costs.
"I have almost run out of ways to describe how outrageous it is.
"Just the interest on the £38m could reopen most children’s centres, restore the youth service, keep Wensum Lodge open or make a big dent in the rat running west of Norwich."
But the county council, while still not revealing its estimate for the cost of the road, said it did not recognise Labour's £400m plus figure.
The authority said some of the money for the project had already been borrowed - although it did not provide a figure.
A spokesman said: "The figures discussed at the corporate select committee were based on the cost of additional future borrowing.
"A large proportion of the local contribution of £37.655m for the Norwich Western Link was borrowed in previous years at the prevailing rate (eg in 2021/22) of 1.78pc, to be paid back over 50 years, costing around £670,000 a year.
"In addition, a portion of this total will be offset by capital receipts which will reduce the overall borrowing required. We, therefore, do not recognise the £403m figure."
The council recently revealed a breakdown of spending on the Norwich Western Link so far - some £40m.
But that breakdown, which included some £36.2m on design, surveys and investigations connected to the road did not outline how much money the authority had borrowed for the scheme.
A council spokesman said: "Borrowing to finance capital projects is a standard procedure and doesn’t change the total capital cost of delivering such projects.
"The Western Link along with the delivery of the much-needed improvements to the A47, will provide a fully dual carriageway orbital route around the city, and in doing so would not only support growth by improving travel for goods vehicles, commuters, emergency services and others, but also take traffic off small, local roads and out of communities."
Comments: Our rules
We want our comments to be a lively and valuable part of our community - a place where readers can debate and engage with the most important local issues. The ability to comment on our stories is a privilege, not a right, however, and that privilege may be withdrawn if it is abused or misused.
Please report any comments that break our rules.
Read the rules here