Part of an new £80m road through Cambridgeshire and Lincolnshire is closed because of structural problems.
The A16 between Crowland to Spalding opened two weeks ago but a four-mile section between Peterborough and Crowland remains closed to traffic.
Cracks have appeared in the surface near Car Dyke bridge at Eye Green.
The road's joint builders - Lincolnshire County Council and Peterborough City Council - could not say when the problem would be fixed.
Lincolnshire senior highways officer Paul Coatup said: "I'm not sure we fully understand the cause of the problem and because of that we're not able to determine the engineering solution and give any idea about the extra cost or delay it might actually lead to."
When the new bypass is completed it will offer an alternative to the A1073 between Spalding and Eye.
At the moment drivers have to use the A1073 between Crowland and Eye Green.
Peterborough city councillor David Sanders said: "This shouldn't have happened in the first place.
This is a huge disappointment to the motorists that have been waiting for years for this road to open to replace the old notorious A1073 and the residents of Eye Green who have been subjected to years of heavy traffic."