A decision to use the £2.25m raised from Norfolk County Council's sale of Carrow House to bankroll the Norwich Western Link has been criticised.
But leaders at County Hall say improvements to the county's road network is vital to cut congestion and support businesses.
County Hall's Conservative-controlled cabinet agreed in August to create a special pot of cash for the 3.9-mile road, made up of money generated by selling off council assets, such as land and property.
That reserve will be used to underwrite some of the council's £30m share of the £198m road, with the council hoping the government will stump up the other £168m for the scheme.
The council's cabinet last week agreed that the £2.25m it got from selling Carrow House to Norwich City Council would go into that pot.
But that prompted anger among political opponents against the road, which would connect the Norwich Northern Distributor Road to the A47 west of the city.
Labour county councillor Terry Jermy said: "The Western Link will consume council funds for many years to come. The road is unaffordable and unjustifiable.
"We've seen significant cuts to council services and large increases in council tax. Additional income like this could be much better spent supporting services such as adult social care and children's services."
Green city and county councillor Ben Price said: "The county council continues to sell off its long-standing assets like Carrow House, and use those capital receipts to prop up the spiralling costs of the Western Link road proposal: a road scheme that will only increase traffic and cause untold environmental damage. This is a travesty.
"This decision demonstrates that the Conservative county council cares little for the citizens of Norwich, or the environmentally fragile Wensum Valley."
Martin Wilby, the council's cabinet member for highways and infrastructure previously said: “In a largely rural county like Norfolk, making improvements to the road network is vital to help reduce congestion, improve road safety, support businesses and reduce emergency response times.
"This investment is even more important now to help the local economy recover from the effects of the coronavirus pandemic."
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