It would be "a tragedy for Norfolk" if the Norwich Western Link is not built, it was claimed, as councillors agreed to borrow millions more to pay for it.
But councillors against the scheme blasted Norfolk County Council's Conservative administration for pursuing a road they described as a reckless gamble.
The estimated bill for the 3.9-mile road, which would connect the A1067 Fakenham Road to the A47 near Honingham via a new road between Weston Longville and Ringland, has gone up from £198m to £251m.
The council hopes to secure £213.4m from the government for the road, but the final £37.7m would need to be found by the council.
Some £3m would come from pooled business rates and £5m from capital reserves, but the rest would need to be borrowed by the council.
Councillors at Tuesday's (July 19) full council meeting agreed - by 41 votes to 12 - to add an extra £7.9m to what it would borrow for the road, taking the total to £29.5m.
Campaigners from Extinction Rebellion Norwich protested outside County Hall and in the public gallery.
Stuart Clancy, Conservative county councillor for Taverham, told the meeting: "If we do not grasp the nettle now, this road will never happen and, if it does not happen, it will be an absolute tragedy for Norfolk.
"The silent majority - the people not in the gallery protesting - want this road and this is the only time we will get to deliver it."
But Terry Jermy, Labour councillor for Thetford West, said: "As well as destroying the environment, habitats and protected species, this road is destroying Norfolk County Council services and its financial credibility. It's time to pull the plug."
Lib Dem group leader Brian Watkins described it as a "big, big gamble", but party colleague Dan Roper, who represents Hevingham and Spixworth, affirmed his support for the road.
Jamie Osborn, Green councillor for Norwich's Mancroft ward, said: "It will be environmentally destructive and entirely reckless for the people of this county."
Andrew Jamieson, Conservative cabinet member for finance criticised what he described as "catastrophist" and "populist" statements opponents had made.
He said borrowing at low interest rates made sound sense and that the road would bring economic growth.
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