From the outside, the green cover looks smooth and fine. But beneath the surface, metal frames and screws are fixed into the walls so that the plastic sheets can be wrapped around the building. A screw was fixed along every metre of the walls.

An architecture professor, who saw it all happen, said the exercise had damaged the historic building.

Woo Pui-leng, who teaches at Chinese University, said the cover was unnecessary. 'After all it's a historic building. Its wall isn't like the wall in your flat. Be it grade one or grade three, if officials are still coming up with a conservation plan, such unnecessary alterations should have been avoided.

'To me, the plastic sheet looks just like an advertisement without much meaning,' she said.

However, a spokesman for the authority said the dents made in the concrete block were reversible.

'The market is not a declared monument. Intervention can be tolerated as long as it is reversible, according to international conservation guidelines,' he said.

The cover was made to make the building look better, he said.

The Antiquities and Monuments Office, which provides technical advice for heritage conservation, had agreed to the screw attachment work, he said.

But a government official familiar with the project said the antiquities agency had raised concern about the plan and the office had suggested other ways to cover the block which did not involve fitting screws or nails.

A spokesman for the office said it required the URA to put the screws in appropriate places and carry out reinstatement work after removing the cover.

The government has proposed a grade three status for the well maintained market, one of the first pieces of modern architecture in Hong Kong built with reinforced concrete.

The market, which ceased to operate in 2003, is one of the few reminders of Streamlined Moderne architecture built in the period. The atrium in the centre of the block is one of the key features of the complex.

While the use of the building has yet to be decided, pending the result of public consultation, the authority had initially proposed dividing the four-storey block in themes, such as a 'cultural square', 'green living' and 'sky garden'.

Possible uses include a bookstore, cyber information points, affordable eateries, healthy lifestyle corners and an arts event atrium.

Lifts and barrier-free facilities would be provided to improve access.

Conversion work will start after a nine-month structural survey is done, and is expected to take five years to complete.

Central and Western District councillor Tanya Chan, who sits on the advisory committee for the revitalisation project, said the committee agreed on the covering work as members thought the market was dirty.

But those living or working in Central may not feel the same way. Amanda Cheung Yin-ting, who lives in Bonham Road, said it would be 'a bit strange' if the reason for the cover was just to make it look neat and tidy.

'It's a good thing that the government has chosen to preserve the market but I would hope it had an intelligent reason for wrapping it up,' Cheung said.

Sinn Digg-kayy, who works in the public relations field, said she would rather see the old building in its glory.

'Even if the damage can be restored, why waste the effort doing so?' she asked.

After all it's a historic building. Its wall isn't like the wall in your flat ... if officials are still coming up with a conservation plan, such unnecessary alterations should have been avoided

Woo Pui-leng, architecture professor at Chinese University

Historic building

The Central Market is one of the few reminders of Streamlined Moderne architecture. It was built in: 1939

Urban oasis

The estimated cost of renovation for the Central Market is, in HK dollars,: $500m