One piece of falling metal pierced the roof of a delivery van, injuring a 48-year-old worker inside. He was treated for hand injuries at Queen Mary Hospital in Pok Fu Lam.

Secretary for Labour and Welfare Matthew Cheung Kin-chung described the fatal accident as "rare and very serious" and promised a thorough investigation. Eight people have now died in the construction industry this year. "This is worrying. We need to step up education, publicity and enforcement on all fronts," Cheung said.

The two people who died had been carrying out maintenance work to the exterior of Tregunter Tower 3. The as yet unidentified man landed next to the bulk of the broken platform. His body was badly crushed.

The other victim, a female subcontractor, fell onto a nearby hillside, her arm and leg severed. The cause of the accident is still unknown and police are investigating together with the Labour and Electrical and Mechanical Services departments.

"We are not ruling out any possibility," a senior police officer said. "We will investigate whether the working platform was overloaded with construction materials or whether it was a mechanical fault."

At about 11am, the metal platform on which the workers were standing dropped suddenly to the 55th floor from a higher floor when its supports apparently gave way. Cement and tools had also been on the platform, according to police.

"A foreign domestic helper told police that she heard a bang before she realised the working platform had stopped at the 55th floor," another officer said. "It split up. Half of the platform broke off and plunged down."

The broken cradle hit scaffolding on the 10th floor, fell another 30 metres and then landed on a podium linking the tower with the neighbouring Hillsborough Court building.

"The whole building came out after the crash," said Andrew Leyden, who lives nearby. "We just saw a massive amount of dust."