The pellets are not toxic in themselves, but environmentalists fear they will absorb toxins which will then pass into the food chain.

A huge volunteer effort in Discovery Bay cleared many pellets at the weekend, but most remain unaccounted for.

Lam said: 'My senior colleagues do feel that in this day and age, when our people are very much concerned about our environment, about ecology, about what they could do more to protect the environment, it would be a better move if we had taken the initiative to notify the public about the incident.'

But she denied there had been flaws in internal communication or that the government had underestimated the risks. 'As far as the follow-up work by various departments is concerned, I think that it has been very vigilant,' she insisted.

The Centre for Food Safety will carry out tests on fish caught by fishermen, some of whom are complaining that fish are showing a poor appetite. As the government tried to ease public concern, green groups and shoppers expressed alarm.

Gabrielle Ho Ka-po, project manager at environmental group Green Sense, said the pellets would absorb insecticides and other chemicals in the sea. 'Some of our volunteers found that some pellets had turned to be yellowish brown, grey or black.

'They are really toxic and if marine animals had eaten them, the consequence would be serious and accumulative,' she said.

Ho estimated it would take HK$200 million to clear up all the pellets. She said Sinopec and China Shipping, owners of the pellets, should pay the fee.

The situation is also alarming shoppers. One housewife at a wet market in Wan Chai was choosing to stay away from local fish. 'Fish sellers will definitely not tell us whether their fish have [consumed] plastic pellets or not,' she said.

But Professor Rudolf Wu Shiu-sun, director of the University of Hong Kong's school of biological sciences and a member of United Nations' Joint Group of Experts on the Scientific Aspects of Marine Environmental Protection, said the pellets were unlikely to have a 'disastrous' effect.

He said there was no definite answer on whether the plastic would absorb toxic matter as it depended on the nature, size and porosity of the plastic. And even if toxins stuck to the surface of the pellets, they would not necessarily be absorbed by fish.

Dr Ho Yuk-yin, a consultant at the Centre for Food Safety, said there had been examples overseas of pellets producing toxin after spilling into the sea, but the process took months.

However, people in the fishing industry fear their livelihoods could suffer. Cheng Ming, who owns a fishing ground in Chi Ma Wan, a bay on the east coast of Lantau Island, said the fish seem to have poor appetites.

'I have picked up about eight to ten packets [of plastic pellets], each weighing about 25kg,' he said.

Kwok Ping-fai, 41, another fishing ground owner in Chi Ma Wan, says fish prices have dropped by 10 per cent. 'Many of us don't know what to do for the best ... it doesn't make sense to cut the fish open to see if they have plastic pellets,' he said.

The government says it has cleared about 23 tonnes of pellets from 16 locations, including 11 tonnes from Lantau and 6.2 tonnes from Lamma. It has also taken 12 fish samples for tests.

Officials said most of the pellets would likely end up in landfill because of difficult to screen them out.