The landscaped plaza at the foot of the Gherkin building in London has been sealed off after a large window popped out of the skyscraper and fell 28 storeys to the ground.
No one was hurt in the accident but rigorous checks are under way on the 180-metre (590ft) tower in St Mary Axe, the heart of the City of London.
A covered walkway has been provided for staff working in the building, which has won a stream of awards for its architects, Foster and Partners, and structural engineers, Arup. Safety specialists are examining 743 similar triangular glass panes and may then move on to thousands of other windows in the elegantly tapering steel, glass and concrete cylinder.
The window fell from an unoccupied floor of the Gherkin which still has 16 of the 40 storeys to let, a year after its official opening last May. Its windows are an important part of its pioneering energy-saving systems which work within a double skin of steel and glass.
Kate Newton of 30 St Mary Axe, the management firm for the Swiss Re bank which owns the Gherkin and uses the lower 15 floors, said the glass cladding had fallen on to the plaza, which was immediately sealed off. The area was deserted at the time.
"We started an investigation at once and we will make sure that everything is fine," she said.
The City of London Corporation said health and safety staff were monitoring the inquiry and were satisfied with the action taken so far.
A slab of wooden sheeting has been rigged as temporary blocking for the hole in the skin on the building, whose windows are opened and closed by a computer in response to the Gherkin's own weather station.
Windows up to the 35th floor are cleaned by a conventional cradle but a hydraulic cherry-picker does the job higher up, including the nose cone staff restaurant.
The accident happened only a week after the Gherkin succeeded in letting one of its spare floors, seven storeys below the affected level, to an international estate agency. The firm joined two financial companies which have a floor each as co-tenants.
Falling windows have long been a key safety issue in skyscrapers, whose design has to take account of constantly differing pressures between the inside of the building and the open air.
One of the most famous "window shedders" was the Hancock Place tower in Boston, Massachusetts, where problems with seals led to dozens of windows breaking on one 60-storey facade.