More than £50m has been added to the cost of the proposed Norwich Western Link road - which would give it a price tag of £251m.
Inflated construction costs - and the need to realign the route due to bats - have pushed the cost of the 3.9 mile road up from the previous estimate of £198m, itself a rise on the original £153m estimate.
However, Norfolk County Council insists there remains "an overwhelming case" for a scheme they say would boost the county's economy, cut congestion and reduce rat-running.
But they also admit, if the government chooses not to cover the bulk of the cost, they could have to consider whether to pull the plug on the controversial scheme.
The price hike would mean the road, stretching from the A1067 Fakenham Road to the A47 at Honingham, including a viaduct over the River Wensum, would cost about £64m per mile.
The war in Ukraine has been partially blamed for hiking material costs, so the allowance for inflation has been increased from £24m to £45m.
But moving the route due to the presence of barbastelle bats has also added costs, including £8.4m on construction costs and £6.8m to compensate land owners for land which is needed for the realignment.
Before the cost went up, the council had been waiting for the Department of Transport to confirm it backed the authority's outline business case for the road.
The council had asked for 85pc of the £198m cost - which would have been £169m.
Now the price tag has risen, council leaders are hoping the government will still cover 85pc - which would be £213.4m.
That would leave the council, which needs to plug a £116m funding gap by 2027, needing to cover the remaining £37.7m.
That is about £8m more than through previous estimates.
Some £29m of the council's share of the cost is currently expected to be paid for through borrowing.
But Andrew Jamieson, the council's cabinet member for finance, said the authority remained committed to the road, which the Conservative-controlled council named as a key priority in 2016.
He said: "As cabinet member for finance, it's my job to make sure a project of this scale is still value for money.
"And, looking at this from the lens of economic infrastructure, this is a key part of what we most firmly believe Norfolk needs in order to level up and for people who need it.
"Yes, it is going to cost a great deal more, but with construction costs rising exponentially throughout the country, it is not surprising.
"And I was pleased to see it still fits the criteria as being high value for money."
Mr Jamieson said he was optimistic the government would understand costs had increased and would agree to cover 85pc of the price.
He said borrowing to cover the rest of the gap would not have any impact on council services.
But he acknowledged should the government leave the council without a considerable gap to plug, councillors might have to consider whether to push ahead.
He conceded: "We would have to look at the scheme very carefully."
Seven options for moving the route were considered, after the impact on bat roosts was identified.
Consultation is planned on a route which the council says avoids direct impact on a maternity colony roost and has a buffer distance from ancient woodland.
Two strategies are currently being considered to reduce the impact on the colony.
The first would be to keep trees in place either side of the road and plant new ones in the centre of the dual carriageway, allowing bats to cross at a safe height, the council says.
The second would be to build a so-called 'bat bridge', which is a structure it is believed bats can use to guide themselves across using their sonar.
A longer, wider, more expensive, viaduct over the Wensum was meanwhile considered, but that option has been rejected.
A council officer said they hoped work on the road could begin in 2023 and open to the public in late 2026.
Further consultation needs to take place before a planning decision is made.
That decision is likely to be called in by transport secretary Grant Shapps, which would trigger months of public inquiry.
And all of that could add even more millions to the cost of the road.
Supporters of the road include the Norfolk and Norwich University Hospital, Norfolk Chamber of Commerce, Norwich Airport, Norfolk Fire and Rescue, the New Anglia Local Enterprise Partnership, Road Haulage Association, First buses, most of the county's MPs and a number of district councils.
But opponents include Norfolk Wildlife Trust, the Norfolk Rivers Trust, the Campaign to Protect Rural England, the Green and Labour groups at County Hall, the Stop the Wensum Link group and Extinction Rebellion.
The county council's cabinet will consider the latest developments around the road when it meets on Monday, July 4.
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