'It is believed that there was natural breeding of [bloodworms] in the compound and investigations will continue tomorrow,' the statement said.

The admission came a day after two pupae and 16 bloodworms - which grow into a mosquito that does not bite humans - were found under artificial turf in the pool area at the Hammer Hill Road facility.

The complex was closed yesterday as hygiene officers, leisure officials and police spent hours investigating. They took water samples and checked artificial turf and rocks around the pool.

The Food and Environmental Hygiene Department also confirmed yesterday that a pupa and 36 bloodworms were found in a water sample taken earlier from the Kwun Tong pool. The department said it had notified the Leisure and Cultural Services Department about that last week.

It was the first time the discovery of pupae, the first stage in a bloodworm's development, was made public officially, amid criticism that leisure officials were hiding the fact that pupal shells were found in the Tai Wan Shan pool.

On Monday, the assistant director of the department, Paul Cheung Kwok-chu, still insisted that no pupae had been found and neglected to mention the discovery of pupal shells. Experts hired by the department have also said the presence of pupal shells and bloodworms was not sufficient evidence to indicate natural breeding of mosquitoes at swimming pools.

Speaking on a radio programme yesterday, LCSD director Anissa Wong Sean-yee said her staff should have disclosed all information to defuse public doubts.

She denied allegations that her staff had destroyed evidence of bloodworms at the Hammer Hill Road pool on Wednesday.

'They have already taken water samples to be tested,' she said. 'The equipment and the leisure pool facilities are still there and the experts can have a look at those.'

Director of Food and Environmental Hygiene Gregory Leung Wing-lup said: 'If there is a suitable environment for the bloodworms then even if these worms are all gone, if they do not handle it properly then there will soon be a second batch.'

A police spokeswoman said they would investigate the case from all possible perspectives.

Kwok Siu-kit, vice-chairman of the Hong Kong and Kowloon Lifeguards' Union, said more than 1,000 bloodworms were found at Hammer Hill but were washed away before police were called. He said he saw at least a hundred of what he believed to be pupae in a bucket of water.