Gabrielle Ho Ka-po, project manager of Green Sense, said: 'A lack of planning by the Housing Department has led to gigantic 40-floor towers, one after another, in public housing estates.

'There is little space between buildings in these housing clusters. As public housing buildings are usually wider than private ones, they can easily affect the air flow in a whole district and spoil its urban views.'

A cluster of nine buildings already existed on the Un Chau Estate, including six 40-floor towers that form a 300-metre-long wall, Ms Ho said.

The three proposed buildings would extend the length of the 'wall' to 400 metres, pending the Town Planning Board's approval, she said.

'It would worsen the wall effect already existent in the district.'

Green Sense suggests there should be a space 40 metres wide between buildings, but it has found distances of just 5 to 15 metres in most public housing estates in Sham Shui Po, Kwai Chung and Tsuen Wan.

On two newly completed buildings on the Un Chau Estate, the gap is just 2 metres.

'As they are so close, one building's air conditioners discharge heat directly into the windows of the other building,' Ms Ho said.

'It causes a vicious circle, with both sides having to turn their air conditioning up.'

Green Sense urged the Housing Department to scale back its latest building plan for the Un Chau Estate and use two nearby plots to meet demand for public housing.

The proposed three buildings are intended for residents from So Uk Estate, which will soon be rebuilt.

'The proposed buildings should be lowered from 40 to 30 storeys,' Ms Ho said.

An official with the Housing Authority said it had conducted environmental design studies for all new projects at the planning and design stages since 2004.

'Studies including micro-climate, daylight, air ventilation and visual impact assessments of the Un Chau Estate Phase 5 project have also been conducted. Results showed that no 'wall effect' will be created,' the official said.

A spokesman for the Housing Department said existing buildings on the Un Chan Estate had been well positioned to avoid the wall effect.

Meanwhile, an application submitted by Green Sense to impose further restrictions on development density on a site in Welfare Road, Aberdeen, was turned down by the Planning Department yesterday.

The group proposed to impose a plot ratio of 6.5 and a distance of 15 metres between the building blocks to allow better air flow in the area.

But the Planning Department said the present 110-metre height restriction would be sufficient to reduce any adverse visual impact on the surrounding area. The Town Planning Board will discuss the application on Friday.