St Clements Hill in Truro will close for almost a week so underground gas and water works can take place.
The road will be closed between 7pm on Sunday 26 May and 7pm on Friday 31 May.
The works, which are taking place as part of the Trafalgar roundabout improvement scheme, are being carried out by CORMAC Solutions Ltd.
Signed diversions will be in place during the road closure with local access retained for nearby residents.
Ian Newby, Senior Project Manager for CORMAC said: "A comprehensive survey of the underground utility network was carried out last summer.
"However, as is often the case, the record of the network did not give a completely accurate picture.
"Unfortunately the reinforced concrete chamber is on the line of the proposed gas and water diversions and has to be removed.
"Combined with the requirement to protect other buried services in the vicinity the only safe way to carry out this task is by closing the road.
"Once the chamber has been removed a suitable route for the water and gas mains can be determined.
"To date we have done all we can to minimise disruption to traffic and have kept full road closures and day time lane closures to an absolute minimum.
"We sincerely apologise for any inconvenience caused and would like to remind people that all businesses will remain open as usual."
For more information call 0300 1234 222.
