Around 3,500 workers at Wembley stadium had to leave the building after a part of the roof dropped 1 metre.
Staff on site said they heard an "enormous bang" and were evacuated before 9am on Monday morning.
The building firm Multiplex say a piece of steel fell to the ground in an area near to workers but no-one was hurt.
Most workers have been sent home for the day. The union which represents the staff has called for a full investigation into what happened.
The beam, which measures about 50m long and 2m wide fell in the northern end of the stadium.
A spokesman said: "It was a rafter in the roof that failed while it was being welded."
The construction of the new Wembley Stadium is already behind schedule. The FA Cup final has been moved to Cardiff's Millennium stadium because it looks likely Wembley won't be finished in time for the 13 May match.
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