The work, required urgently to meet Health and Safety Executive demands, has been planned jointly with Cheshire County and Chester City councils, emergency services and public transport operators.

One-third of a mile of main will be renewed from Mill Street, Handbridge, over the Old Dee Bridge along Lower Bridge Street to its junction with Pepper Street and Bridge Street.

One of the city's best known landmarks, the 14th century scheduled ancient monument will close until September 1, but remain open to pedestrians and cyclists who may have to dismount.

More than 10,000 residents and businesses in the affected areas have been told about the scheme, traffic diversions and changes to bus routes via a door-to-door leaflet drop.

Static and variable message signs on affected routes will warn motorists of the work and consequent diversions. And a web page at www.cheshire.gov.uk/roadworks will give regular updates on the scheme's progress.

Craig Horrocks, National Grid Contract manager, said: "The work has to be timed for the summer months when demand for gas is low. Engineers can then isolate sections of main and insert the new plastic pipe, avoiding the need for open excavations and turning off supplies.

"We apologise for the inconvenience our work may cause. There will be some disruption in the short term but the long term benefit will be a continued and safe gas supply for Chester homes and businesses."

Preliminary work, which will not affect traffic, began on July 28 and the three-phase project is expected to take until mid-November, unless work reveals the need for archaeological surveys.

Cllr Eveleigh Moore Dutton, Cheshire's executive member for highways and transport, said: "Such projects within a city plan laid down by the Romans are always going to cause some disruption to 21st century traffic.

"But as with the closure of Hoole Bridge we will be working together to minimise disruption, both for our residents and visitors."

Cheshire County Council's Highways Department will monitor the situation – including any effect on the Grosvenor Bridge – through its traffic monitoring system and will take action, if appropriate.

Built around 1387, the Old Dee Bridge replaced wooden structures built by the Romans, which were frequently washed away.

Its structure has remained largely unchanged since the time of Richard II, who actually crossed it as a prisoner on his way to the Tower of London.

In 1999, core samples taken from its sandstone arch barrels by county council engineers, revealed that their Plantagenet counterparts had met the requirements of the European 40-tonne weight limit.