No injuries were reported as steel workers from Australian-based Mutliplex Construction Group were sent home with full pay on March 20.
Tom Kelley from the GMB union, which represents the steelworkers on the project, said that after hearing a loud bang, the workers ran from the stadium. Kelley urged a full investigation and said to reporters: "After anything like this, where there could have been a death or injury, there has always got to be a full and independent inquiry."
The steel rafter, which was held in place by a temporary connection, slipped 20 inches as it was being welded in place, but no damage was done to the roof, according to media reports.
Workers were not allowed to return to the stadium until March 21, after engineers from Multiplex said it was safe for them to go back to work after the rafter was secured. They reported that the incident was an isolated one that hasn't occurred in any other part of the structure.
A spokesperson from Mutliplex also said that the company would be conducting an investigation with its engineers and subcontractors and examining whether the incident had any long-term implications for the project.
No word has been given about the delays the accident may bring to the completion of the stadium and its costs. A GMB union spokesperson had said on Jan. 30 the steel workers would have the work completed by March 31, but since the accident, "all bets are off" about the completion deadline.
The U.K. Health and Safety Executive has been informed of the problem.
In January 2004, one construction worker was killed and another injured at the Wembley site when scaffolding collapsed and dropped about 100 feet.
Built on the site of one of Europe's best-known soccer grounds and where England had its only World Cup victory, the 90,000-seat stadium which has been in construction since September 2002 has a 133-meter- (436-feet-) high steel arch that's visible across much of London.
The new Wembley Stadium will host matches of England's national soccer team, league playoffs and concerts by bands such as the Rolling Stones.