According to a contact they are required to sign, male workers may not have any contact with Israeli women - including prostitutes, a police spokesman, Rafi Yaffe, said.

He said there was nothing illegal about the requirement and that no investigation had been opened.

An Israeli lawyer who did not want to be named said while the contract might appear legal, it would be rejected if challenged in court. "The point is that a Chinese worker will agree to anything and then will not have anyone to help them if there is a problem," he said.

The labourers are also forbidden from engaging in any religious or political activity. The contract states that offenders will be sent back to China at their own expense.

About 260,000 foreigners work in Israel, having replaced Palestinian labourers during three years of fighting. When the government first allowed the entrance of the foreign workers in the late 1990s, ministers warned of a "social timebomb" caused by their assimilation with Israelis.

More than half the workers are in the country illegally. Israeli police have increased efforts to deport those working without permits because of rising Israeli unemployment, which has reached 11% in recent months.

Advocates of foreign workers, who also come from Thailand, the Philippines and Romania, say they are subject to almost slave conditions, and their employers often take away their passports and refuse to pay them.

Analysts say there is much division within Israeli society over immigration and status, although the conflict with the Palestinians has given it an appearance of unity. Recent immigrants such as Russians and Ethiopians are disliked by older immigrants, and there is much resentment among secular Israelis at the privileges given to ultra-orthodox Jews. The foreign workers are at the bottom of the pile.