Preston road users must wait until February 2022 for the Corporation Street road closure to end, according to Lancashire County Council.
The council said the extension was necessary after discovering large sand deposits underneath the carriageway.
It means they have to change the design of the foundation to ensure the new road is strong enough to support its use as a bus route.
Read more: Corporation Street roadworks and closures planned as part of Ringway scheme
A Lancashire County Council spokesperson could not confirm a completion date but said it is now a more significant job than resurfacing and will take longer to complete, with at least the end of January expected.
Access to all car parks will be maintained to allow shoppers to access the city, and there are marked diversion routes.
The work has started on Corporation Street North, and there will be full and part road closures until February 2022 while bus services move to Friargate.
Read more: Watch how Friargate and Ringway in Preston city centre to look after £14.7m revamp
During November, the council closed Corporation Street between Marsh Lane and Heatley Street to construct bus stops.
Corporation Street North will be reopened with narrowed carriageways and temporary traffic signals between Heatley Street and Ringway.
The measures will continue until the Spring when Heatley Street will need to close again to safely improve and surface the footways and carriageway on the narrow road.
Read more: Delays likely as major Friargate and Ringway roadworks to begin
This second closure will last for about three weeks.
The council thanked residents for their patience during work to reconnect both sides of Preston.
They said the work is facilitating better leisure and shopping and will improve facilities for sustainable travel.
For more information on alternative travel routes, visit – www.lancashire.gov.uk/fnr
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