Traffic chaos is expected to hit the streets of Belfast today after the mystery movement of an underground air bubble caused one of the city’s busiest roads to collapse.

Motorists are bracing themselves for up to a week of major delays after tarmac at Cromac Street sagged and left a hole which could stretch up to 15sq metres in size.

The road is not expected to open until the weekend, and an emergency meeting was held yesterday in an attempt to minimise disruption for commuters.

Located close to the city centre, Cromac Street is one of the main arterial routes for commuters coming in from south and east Belfast.

The section between East Bridge Street and May Street has been closed and drivers have been warned to expect considerable congestion.

Motorists have been advised to avoid the area altogether if possible.

Roads Service spokesman Colin Brown said the unusual problem arose because the road is built on a deposit of material known as “Belfast Sleech”.

“Periodically we can get voids forming under the roads,” he said. “Sometimes it's very clear what has caused it, other times it's quite a mystery.”

The rupture occurred above a storm-water tunnel which is part of the £120m Belfast Sewers Project. It is thought to have been spotted by a passing bus driver early on Saturday morning.

An underground air pocket is believed to have become disturbed and risen to the surface. The tarmac mostly stayed intact, but a 15sq metre hole emerged underneath.

Bill Gowdy from Northern Ireland Water said the air pocket was probably disturbed a year ago during work on the sewers project.

“The tunnel is being driven under public roads so there was never any risk to buildings in the area,” he said.

“It was a very unusual event which occurred after a rare chain of events and is unlikely to happen anywhere else in Belfast.

“We have dug out the hole and hope to have it filled in by the weekend. The work is continuing 24 hours a day and we will do our best to have the road open quickly.”

Mr Gowdy said the new storm-water tunnel lies 15 metres beneath the surface and was not damaged.

“The tunnel has not been affected and is still on course to begin operating before the end of the year,” he added.

In a statement, NI Water apologised for any inconvenience caused to the public.

“NI Water staff have been on site overnight and working with the other utilities to establish the extent of the problem, fully determine the cause of the road depression and to begin remedial action to make repairs,” it said.

Alex Maskey, a Sinn Fein MLA for South Belfast, visited the scene yesterday and said everything possible was being done to repair the road.

“I met with officials and took the opportunity to commend them for their speedy response,” he said. “I also raised my concerns about the inevitable traffic disruption particularly as we head into the working week.

“They have assured me that they are doing everything they can to rectify the problems and get things back to normal as soon as possible.

“It was clear from my visit that a lot of work has now begun to repair the damage caused to the road.”

The Roads Service has signposted diversion routes while Cromac Street remains closed.

“Those travelling from the east are advised to use Queen’s Bridge instead of Albert Bridge,” a spokesman said. “Those travelling from the Ormeau Road to the city centre or Victoria Street should use Ormeau Avenue, Bedford Street or Howard Street, but it is going to be very heavily congested.”

Translink said that citybound East Belfast Metro services in this area are diverted via Short Strand and Queen’s Bridge. Citybound Ulsterbus services are also diverted via Short Strand and Queen’s Bridge.