M4 motorway London to Heathrow section closed Published duration 7 July 2012

media caption The BBC's Tim Reid says the normally congested motorway is now empty

The main motorway link from London to Heathrow Airport has been closed to all traffic after a crack was found in a "sensitive area" of the road structure.

The M4 is shut between junctions one and three and is not expected to reopen until Thursday morning.

Large vehicles were already banned from the elevated section, known as the Boston Manor Viaduct.

The Highways Agency said it expects all work to be finished before the start of the Olympic Games later this month.

The full closure of the motorway in both directions between Chiswick and Feltham began at 20:00 BST on Friday.

Vital route

Restrictions on vehicles weighing more than 7.5 tonnes had been in place since March when hairline cracks were discovered in some of the steel beams.

The agency said on Friday workers "found a further crack in a sensitive location which requires us to keep the viaduct closed until the repair is complete".

Jon Caldwell from the Highways Agency warned there would be delays.

Diversions have been put in place along the A312 and the A4, and the A40 is expected to be congested as well.

"The repair works that we need to do, to bolt the plates to the decking, needs to be done with no traffic on it," Mr Caldwell told the BBC.

"We need to do it now so that it's all completed ready for when the Olympic traffic starts to use the road in a few weeks' time."

The M4 is the main route between Heathrow Airport and central London and will be vital for transporting visitors into the city for the Games.

It is also the major road link from London to the West Country and Wales, as well as being a heavily-used commuter route.

London Mayor Boris Johnson said it was vital the repairs were completed "as speedily as possible."

Just before Christmas, the Hammersmith Flyover - part of the same route in and out of the capital - was closed for five months after defects were found with its structure.