Officers from the Buildings Department also inspected the 71-year-old site yesterday. It confirmed that there had been no structural demolition so far.

Mrs Lam said she had urged the Antiquities and Monuments Office to quickly review whether King Yin Lei should be granted monument status.

Damage to the building could be seen on the outside. The red brick walls, one of the most distinctive features of the mansion, were pitted with dents believed to have been made by hammers or axes.

On Tuesday and Wednesday, many of the green tiles on the roof of one of the city's oldest and best-preserved mixed-style mansions were removed, and the three Chinese characters 'King Yin Lei' on a plaque at the gate were also destroyed by jackhammers.

Only structural changes to a building are covered under the Buildings Ordinance, which has no power to prevent other major non-structural work.

Albert Lai Kwong-tak, chairman of the Hong Kong People's Council for Sustainable Development, said the issue was a 'huge grey area'.

'The buildings ordinance only covers structural change of a building. But then we are talking about a unique building with historical value. Any damage would ruin its value,' he said. 'The government must save the mansion now by whatever means possible.'

After meeting Director of Buildings Cheung Hau-wai yesterday afternoon, Democratic Party chairman Albert Ho Chun-yan said: 'What the new owners have done is vicious. While so many people came out to ask the new owners to stop ruining the mansion, it seems [they] deliberately damaged it to an extent that it is difficult to restore it to the original condition.'

Five Buildings Department officers were initially refused entry by security guards to the site when they tried to inspect it at 11am yesterday. After about 45 minutes, the guards finally allowed four of them in, apparently after consulting their senior. The officers declined to comment when they left after about an hour.

Built in 1936, the mansion's delicate oriental design and craftsmanship attracted the attention of conservationists and tourists, but it has not been included in a government list of historical building requiring conservation. A spokeswoman said last night that the Antiquities Authority was closely monitoring the mansion work. 'However, it is still not known if a meeting over the heritage status of the structure will be held,' she said.

A neighbour said: 'The mansion is a tourist spot. I've lived here for more than 20 years and seen many foreigners coming here to take photographs. It's a pity. I don't think anything can be done now.'

The property, previously owned by businessman Stephen Yow Mok-shing, was sold for HK$430 million to the British Virgin Islands-registered ICE Wisdom Limited on July 7.