Scottish Water say Nelson Mandela Place and West George Street, between West Nile Street and George Square, will remain closed to traffic except for taxis and service vehicles for a further two weeks.
It had originally been hoped that the works, which began on January 21, would be finished by today.
Now they look set to continue until February 25.
Water chiefs say the continued disruption is down to the discovery of a burst main and a collapsed disused sewer which caused subsidence near Queen Street.
Mark Maclaren, Scottish Water's regional community manager, said: "The extension of the road closure is required to enable us to repair the burst main and the collapsed sewer.
"We would like to apologise for the inconvenience.
"Because the burst water main and sewer collapse occurred while we are carrying out our water mains upgrade in Nelson Mandela Place and West George Street we were able to respond quickly, investigate and establish the cause of the subsidence and start the necessary repair work."
The £3.5million city centre water mains upgrade, which started in July 2012, will reduce the risk of bursts and the inconvenience to customers, road users and pedestrians.
It will also help provide clearer, fresher drinking water to properties in the area.
West George Street from its junction with West Nile Street to its junction with Dundas Street will be opened to taxis and service vehicles only.
The short stretch of West George Street from Dundas Street to the junction with George Square will remain closed to all traffic, including taxis and service vehicles.
These vehicles will be diverted to St Vincent Street.
Other local diversions will remain in place.