Detectives said the scale of the exploitation of the immigrant workers was “appalling” and revealed they were closing in on the Merseyside gangmasters suspected of sending the immigrants to their deaths.

Detective Superintendent Mick Gradwell, who is leading the investigation, said: “The wages for nine hours’ hard graft were £1. They were ill-equipped to go on the beach and they went at the wrong time.”

The tragedy has highlighted the plight of illegal immigrants who work in the black economy. David Blunkett, the home secretary, said yesterday that China urgently needed to change its policy to allow asylum seekers rejected by Britain to return home.

The victims of Thursday’s tragedy, 17 men and two women, were among a group of immigrant workers trapped by the incoming tide while harvesting cockles in the bay.

Rescue services were alerted after a passer-by spotted the silhouettes of the trapped workers on the sandbanks. One survivor was pulled from the water and 15 others managed to scramble to shore before the tide engulfed their colleagues. Last night police gave up hope of finding any more survivors.

Gradwell said his team was “narrowing down” the names of gangmasters who might be responsible and were intending to speak to two men, both white, based in the Liverpool area. “We have sent a team to Merseyside and they will be looking at who arranged the transport,” said Gradwell. “We urge any gangmaster involved to come forward.”

A number of addresses in Liverpool were searched yesterday. A police spokesman said he was not aware of any arrests.

Brian Wong, a leader of the Liverpool Chinese community, said workers arrived in Britain deeply in debt to traffickers and were willing to work for gangmasters whatever the wage.

He said the agricultural wages in Fujian province in China were only £3 or £4 a month: “They think it’s going to be easy, but when they arrive they find it impossible to integrate into society.”

Most of the workers speak little or no English and are unlikely to find work other than that offered by the gangmasters. There have been previous reported cases of immigrant workers paid less than £2 a day after deductions for transport and accommodation.

Survivors of the tragedy were yesterday being interviewed by police with the help of interpreters. Nine of them are known asylum seekers, five have had no previous contact with immigration authorities and two are Europeans.

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