The Norwich Western Link is now unlikely to open until 2029 at the earliest, officials have admitted, as they revealed the cost of the road has risen by almost 10pc in the last year.
Leaders at Norfolk County Council pledged they remain committed to the scheme, having been boosted last month when the Department for Transport agreed to fund £213m towards the project after scrapping part of the HS2 rail route.
But officers at County Hall confirmed the cost of the 3.9-mile road has risen to £273.9m from the £251m estimated last summer.
The council is optimistic the government might yet add more money to cover the £251m, having signalled it might increase the sum.
But, if the government sticks with the £213m, then County Hall will have to plug a gap of more than £60m and would need to find about £23m if Westminster does increase its share to £251m.
That could mean the council has to borrow millions of pounds or dip into County Hall's reserves - at a time when the authority is facing making £46.2m of cuts and savings.
Graham Plant, the council's cabinet member for highways, infrastructure and transport, acknowledged the Conservative-controlled cabinet would have a decision to make if it does not get more from the government.
But he said: "From my point of view, I would be pushing to complete it. I am pretty confident the ruling group would be in support of the project. There's an element of realisation that this scheme is imperative for Norfolk going forward.
"It’s crucial that we continue to invest in Norfolk’s transport infrastructure, to tackle existing traffic problems, support local businesses and enable our road networks to cope with planned housing and job growth."
The road, which would connect the Northern Distributor Road to the A47 west of Norwich, will now cost more than £70m per mile.
Council officers said the delay in the government agreeing the outline business case for the road meant inflation had pushed up the bill.
And surveys have established building the viaduct over the river Wensum, which is a crucial element, will be more complex - and costly - than first thought.
Officers said the softness of the ground means a special temporary platform will have to be built to enable the beams of the bridge to be hoisted into place, adding to the cost and extending the construction period.
That means if the scheme does secure planning permission and get through a potential public inquiry, work would start in summer 2026, with the road open in 2029.
When the county council first lodged the initial business case in 2019, it had hoped it would be open in 2025.
The latest developments will be discussed at a meeting of the council's cabinet next month, where councillors will be asked to agree steps to pave the way for an application to the council's own planning committee early next year.
A legal challenge could yet trigger a public inquiry, which would further delay the project - and potentially increase costs further.
And critics said the council ought to scrap the scheme, which the council has so far spent £45m on.
Emma Corlett, deputy leader of the Labour group at County Hall, said: "Just servicing the £60m council contribution is the equivalent of more than 1pc increase on council tax for every family in Norfolk for decades.
"There is a bottomless pit of yet more inevitable overspends. Inflation will add more and more, and the planning application isn't even in yet so the risks are vast.
"In the middle of the current cost of living and budget crisis it now beggars belief."
Council officers have also conceded the road will lead to increased carbon dioxide emissions, having previously said it would reduce such emissions because it would cut the distances vehicles would travel.
The admission could leave the scheme open to judicial reviews from environmental campaigners.
Green county councillor Jamie Osborn said: "After years of denial, the county council has been forced to U-turn on previous claims and admit that the Norwich Western Link will add thousands of tonnes more carbon pollution into the environment each year.
"The Norwich Western Link is a disastrous white elephant and should be stopped before any more money is wasted."
Liberal Democrat group leader Brian Watkins said: “There must be serious doubts about whether the Norwich Western Link will ever go ahead now that it's been put back to 2029.
”I’m not really surprised with this.
”However, I will be surprised if costs remain at £274m.
“I think it's inevitable that it will be far higher even if government pledge more funding towards it.“
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