Three long-awaited projects to improve the A47 have been thrown into doubt after the High Court agreed to consider claims by climate campaigners that the schemes should be scrapped.
The decision means that the works - including seven miles of new dual carriageway - will definitely be delayed and could be ditched completely.
The schemes involve the revamp of Thickthorn roundabout on the edge of Norwich and dualling of stretches of the road between Blofield and North Burlingham, plus Easton to North Tuddenham.
Work was originally intended to start in 2020, but after a number of delays it was hoped the projects could finally get under way next year.
The schemes were granted permission by the Department for Transport after recommendations by planning inspectors, following months of public hearings.
But a challenge against those decisions was made by climate activist Andrew Boswell, a former Green city and county councillor.
He argued the impact of carbon emissions that would result from the schemes had not been properly addressed.
The court has now agreed to hear the case and judges will consider the evidence to decide whether the decisions to approve the schemes were lawful or not.
Chris Griffin, programme leader for National Highways in the East Region, said: "While we are disappointed with the judgment, we stand by our plans and remain committed to investing almost half a billion pounds on this series of improvements to the A47 across the east of England.
"We are now discussing the implications with our legal team and will release more information in due course."
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Mr Boswell said: "I am very pleased that we are going to go to the next stage and the issues will now get a full hearing in the High Court.
"Our grounds for challenge include that the impact of carbon emissions that would result from the scheme have not been lawfully assessed.
"They salami-sliced the schemes and did not take into account the cumulative carbon impact of the three schemes - and also the Norwich Western Link.
"And we are also arguing that the licensing relating to the Norfolk super-colony of barbastelle bats was not done correctly.
"The North Tuddenham scheme is just a hop away from that super colony."
A date for the hearing, which is likely to last about a day and a half, has yet to be announced but it is likely to be in the spring or summer of 2023.
Mr Boswell, who has raised almost £20,000 towards the costs of the challenge through a crowdfunding campaign, said: "The hearing is the costly bit and we will continue to raise funds."
What are the schemes?
The A47 schemes were originally announced in 2014, by then prime minister David Cameron.
There was £300m earmarked for Norfolk schemes, along with other improvements in Cambridgeshire.
Mr Cameron had said work would start by 2020, but the three biggest Norfolk projects have only recently been granted development consent.
The plans for Thickthorn include a new slip road off the A11 northbound, which will take motorists beneath both roads before re-joining traffic on the A47 heading towards Great Yarmouth - eliminating the need to use the Thickthorn roundabout.
The changes would also see a segregated left-hand turn added to those travelling eastbound on the A47, a new footbridge and a fourth lane on the southern part of the junction.
Just over 1.6 miles of the road between Blofield and North Burlingham - considered a crash blackspot by police - is due to be dualled.
Five-and-a-half miles of the road between Easton and North Tuddenham is also due to be dualled.
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