Talks over how millions of pounds are being added to the bill for the Norwich Northern Distributor Road will take place today - behind closed doors.
Bosses at Norfolk County Council have confirmed that delays and the need for changes to the work on the £178.5m road have pushed up the price.
But, with the council set for negotiations with contractors Balfour Beatty over the extra costs accrued, they are not saying how much more could be added to the bill.
The public and press will be excluded from a meeting of the council's environment, development and transport committee when the financial situation is discussed.
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The council says that the public interest in keeping commercially sensitive information under wraps exceeds the public interest in releasing it.
In September, council bosses admitted the cost of the road, which will stretch from the A47 at Postwick to the A1067 Fakenham Road, could increase by £6.8m.
But at today's meeting, councillors will be told how delays and changes, particularly around the Rackheath railway bridge where design changes were needed because of Network Rail's requirements, mean millions more has potentially been added to the bill.
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The council is not revealing the exact figure, saying it needs to be kept private because of negotiations with contractors Balfour Beatty.
Tom McCabe, executive director of community and environmental services at County Hall, said: 'This is the subject of robust commercial discussions between us and Balfour Beatty.
'It's my job to make the figure as low as possible. The reason we have a below the line report is there will be commercially sensitive information in the report.
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'It's only right we share that with councillors, but it's important that we protect our position.'
But Denise Carlo, Green Norwich city councillor and a long-standing opponent of the NDR, said: 'They should be discussing it in public because this is public money. If they have to spend more, then it is going to have to come from somewhere.
'I think them discussing it below the line is more to do with embarrassment and the lack of ability to have forseen these extra costs.'
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