The work was expected to start today but officials said it had been put back until Wednesday.
National grid says it will take five weeks to replace the existing iron gas mains, which have been in use for many years, with hardwearing modern plastic pipes which are designed to last for 80 years.
It said it will have to close the Saturday Market Place to through traffic for the first three weeks of the project. Work is due to start on Monday.
Access will be maintained for businesses and two signed diversion routes will be in place. The first will be via College Lane, South Quay, Purfleet Place, King Street, Tuesday Market Place, St Ann's Street, North Street, John Kennedy Road, Austin Street, Blackfriars Road, St James' Road, Millfleet, Stonegate Street and Church Street.
The second will be via Church Street, Stonegate Street, Millfleet, St James' Road, Railway Road, John Kennedy Road, North Street, St Ann's Street, Tuesday Market Place, King Street and Queen Street.
For the final two weeks, the Saturday Market Place will reopen to vehicles and temporary traffic lights will be used to control traffic flows. There will be access on foot to businesses throughout the project.
Anyone with queries about the work and how it will affect them should call 0800 096 5678.
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