Castlegate in Grantham has been closed to traffic with immediate effect following the discovery of subsidence in the road.
A small hole, just a few inches across, has appeared in the road close to the junction with Bluegate.
Lincolnshire County Council highways and Anglian Water are investigating the problem ahead of repair works beginning tomorrow (Thursday).
Anglian Water will carry out a closed circuit television survey of the existing sewer under the road.
The road has been closed between south of the Bluegate junction and north of Middlemore Yard junction.
The closure will be removed as soon as the cause is established and repair works are complete, says the county council.
Local access will be allowed, with a diversion via Bluegate, Swinegate, Watergate/High Street and Finkin Street.
Through traffic should avoid the area if at all possible.