Workers had a "lucky escape" today as a 50-tonne steel roof rafter collapsed at the new Wembley Stadium.
The accident sparked fears of further delays to the stadium as engineers tried to discover what went wrong.
Workmen were evacuated and sent home for the day following the incident at about 8.30am.
Engineers said rafter No 24 on the north roof collapsed without warning, falling about one metre, as several workmen were working on it.
One engineer who witnessed the accident said: "I heard a really loud bang and looked round and saw the lads running away.
"There were about three people really close to it, they were beside it working on it at the time and they had a very lucky escape.
"What seems to have happened is that there is a hook where the rafter fits on to the leading edge beam and that's collapsed - the weld wasn't there or wasn't finished. Obviously, checks on other parts of the roof are going to have to be made."
Another worker said: "One of those rafters weighs about 40 or 50 tonnes so it's a massive thing to fall.
"It will definitely lead to delays but it's hard to say how long that will be.
"We have to get it back up and find out what happened."
No one was hurt in the incident but union officials have called for a full investigation.
Multiplex managing director Martin Tidd described the incident as "minor" and said he hoped workers would be back on site tomorrow.
Engineers were assessing the situation but Mr Tidd said the roof structure had not been affected.
Early indications suggested that the roof rafter in the north west of the stadium had moved 20 inches.
Workers, including steel erectors, were "relatively close" to the incident.
"An investigation is under way and we are looking for a solution this afternoon. The indications are that it is not a design fault," he told a news conference.
The rafter involved in the incident was supported by temporary steel connections, said Mr Tidd, adding that nothing such as this had ever happened before at the site.
He denied that safety was being sacrificed to complete the project, and said one other beam similar to the once involved in the incident was now being checked.
Multiplex said in a statement: "At 8.35am this morning, we evacuated Wembley as a precaution following an isolated incident on site.
"Our early investigations into this incident have suggested that a roof rafter of approximately 30 metres on the north west roof structure has moved circa 500 millimetres at one end from its intended permanent position whilst being held by temporary connections.
"Engineers are currently assessing the situation and once this investigation has been completed, plans can be finalised to relocate the rafter.
"This has not affected the structure of the roof or the stadium, neither of which have moved as has been reported.
"At this point we believe that this is a minor incident, however as Multiplex has rigorous health and safety guidelines, we have taken the cautious measure of evacuating the site of the 3,000 plus workers until we have a full report on the incident.
"Once these investigations have been completed and the engineers have formulated a methodology for resolving this issue, we will be able to provide a further update."Reuse content