The groups say the growing number of pet hoarders is also a symptom of the government's failure to address the problem of homeless animals, forcing people to take matters into their own hands.

Both the Society for the Prevention of Cruelty to Animals (SPCA) and the Society for Abandoned Animals (SAA) say that pet hoarding started becoming a serious issue two years ago as the number of abandoned animals increased.

But a spokesman for the Agriculture, Fisheries and Conservation Department said it conducted regular operations to catch strays and did not encourage the public to pick up homeless animals.

SAA co-founder Noel Fan Wan-ching said she knew of at least 20 private shelters - double the number there were two years ago.

'They might have 10 to 20 animals initially, but now many hoarders have more than 100 cats or dogs,' Ms Fan said.

Fiona Woodhouse, SPCA deputy director for welfare, said her group knew of dozens of private shelters and believed they were just the tip of the iceberg.

She blamed the growing problem of abandoned animals on the pet ban in housing estates, dumping of guard dogs by construction sites and impulsive pet buying.

On the positive side, Ms Woodhouse said society was generally becoming more caring about the plight of animals, which had led to more people trying to help them.

Peter Lin Chun-hung, SPCA's head of homing, said private animal shelters were often found in disused warehouses, pig pens and homes in villages in the New Territories and the outlying islands. Most keep the animals in cages.

Some carers may have a dozen animals or more in their tiny homes, he said, citing the case of a woman who keeps 60 cats and dogs at her 400 square foot flat in Wan Chai.

One of the unfortunate side-effects is that the health of carers sometimes suffers as regular social contact with people diminishes and they begin worrying more about their pets. Ms Fan said she knew of cases where carers developed depression and mental illnesses as a result.

'They don't communicate with others and isolate themselves. They won't listen to advice and just follow their own ways,' she said, adding the animals sometimes suffered as a result. 'They might be held in small cages and are never let out for exercise.'

Most private shelters are operated by women, some of whom have spent all their savings and run up debts to feed the animals.

Ms Fan recalled the case of one woman who left her Kam Tin shelter for a month when she ran out of money, leaving 200 starving dogs to kill each other for food.

'When the operators start out, they are happy as they feel they can help the animals,' she said. 'But after a few years they start to lose control. They can't rest and spend all day and night cleaning and feeding their pets.'

Ms Woodhouse praised the shelter operators for filling a valuable role, but criticised the lack of rehoming and the inadequate care given to the animals. She had seen paralysed dogs left without help, for example.

Mr Lin said he also had seen some 'horrific scenes'.

In one case, he said, dogs were left with no water for a week. In another shelter, caged cats were stacked on top of each other like office files. The lack of neutering and spaying in the shelters meant that interbreeding was common, leading to many deformities in the offspring.

The SPCA started a programme in 2002 under which pet food, medical treatment and the offer of rehoming animals were extended to desperate owners of shelters.

But Mr Lin said many dogs living in these conditions rarely interacted with people when moved to a different environment, making it difficult for them to be re-homed.

The SPCA and SAA, which both rely on public donations, said they did not know how to provide the best assistance to animals in shelters.

'I don't want to report them to Agricultural, Fisheries and Conservation Department as the animals would be put down, but we can't take the animals either as our shelter is full,' said Ms Fan, whose group provides free neutering and spaying services for the shelters' animals.

Ms Fan and Ms Woodhouse urged the department to regulate breeders to control animal populations and follow the footsteps of other countries that use a trap-neuter-release approach to tackle the problem.

The spokesman said the department was evaluating the trap-neuter-release option, but could not say whether it would be implemented.