Race against clock to fix vital transport artery for Olympics

London faces a race against time to repair a crucial flyover which will carry traffic into the capital for this summer's Olympics, according to one of Britain's leading structural engineers.

Click here to see the 'Hammersmith flyover- High chance of disaster'

The Hammersmith Flyover, which carries 90,000 vehicles a day on the A4, the road between central London and the West, including Heathrow airport, has been closed for a fortnight after serious defects were found in the 50-year-old structure. Major traffic congestion is already being caused, which is likely to increase when many schools go back this week.

The cables which squeeze together the separate pre-cast concrete segments of the bridge, known as pre-stressing cables, were found to have corroded because of water damage and to have lost much of their tension – a problem which, if not dealt with, would lead to the flyover collapsing.

Engineers say several other road bridges in the UK are threatened by the corrosion process known as chloride contamination, caused when salty water seeps into concrete when ice melts. Spaghetti junction (Gravelly Hill) near Birmingham was afflicted with chloride contamination and in 2010 underwent repairs costing £2.7m.

In Hammersmith, although the corrosion can be temporarily repaired for the Games, the job is going to be complex and lengthy, according to Dr Chris Burgoyne of the Department of Engineering at Cambridge University, who has been called in by Transport for London to advise on the problem.

Asked whether it would be a race against time to get the flyover reopened in time for the 2012 Games, which begin on 27 July, Dr Burgoyne said: "Yes. It will take a long time to sort it out. You're definitely talking about months."

It is now dawning on the organisers of the Games, and in particular the Mayor of London, Boris Johnson, that the state of the bridge is presenting a serious threat to the complex transport plans drawn up to allow athletes and visitors to move around the capital during sport's greatest spectacle.

On Friday Mr Johnson visited the flyover to inspect the work and was at pains to insist that all would be well. "One thing I can assure Londoners of is a plan is being finalised within the next few days and work is already beginning on strengthening the flyover so it is fully operational well ahead of the 2012 Games," he said.

But Dr Burgoyne, who is reader in concrete structures at Cambridge, and who agreed with TfL's consultants' advice to close the flyover immediately when he learned of the damage on 23 December, explained just how difficult it is going to be to deal with the damaged cables inside the 622m-long structure, which opened in 1962. "These days, in building a pre-stressed concrete structure, you would leave the cables exposed all the way along their length. It means they're slightly more liable to corrode, but it means you can inspect them easily and replace them," he said. "But what they did in Hammersmith is they surrounded them with mortar boxes, which effectively stop you seeing what's going on in the cable. It means you can't easily replace the cables.

"It's a big job. A lot of the work would have to be done inside the box sections and access is not easy. I couldn't stand up... at mid-span it's only about 4ft high, so working conditions are quite cramped in there. You can't throw lots of men at one location – there physically isn't room for them." He added: "They've got to come up with a design, they've got to get it checked, and get it approved."

Transport for London has been working around the clock on the site. The investigation will continue this week before it decides if the flyover is strong enough to reopen even to light traffic.