Husbands with multiple wives have been given the go-ahead to claim extra welfare benefits following a year-long Government review, The Sunday Telegraph can reveal.

Even though bigamy is a crime in Britain, the decision by ministers means that polygamous marriages can now be recognised formally by the state, so long as the weddings took place in countries where the arrangement is legal.

The outcome will chiefly benefit Muslim men with more than one wife, as is permitted under Islamic law. Ministers estimate that up to a thousand polygamous partnerships exist in Britain, although they admit there is no exact record.

The decision has been condemned by the Tories, who accused the Government of offering preferential treatment to a particular group, and of setting a precedent that would lead to demands for further changes in British law.

New guidelines on income support from the Department for Work and Pensions (DWP) state: "Where there is a valid polygamous marriage the claimant and one spouse will be paid the couple rate ... The amount payable for each additional spouse is presently £33.65."

Income support for all of the wives may be paid directly into the husband's bank account, if the family so choose. Under the deal agreed by ministers, a husband with multiple wives may also be eligible for additional housing benefit and council tax benefit to reflect the larger property needed for his family.

The ruling could cost taxpayers millions of pounds. Ministers launched a review of the benefit rules for polygamous marriages in November 2006, after it emerged that some families had benefited financially.

The review concluded in December last year with agreement that the extra benefits should continue to be paid, the Government admitted. The decision was not publicly announced.

Four departments - the Treasury, the DWP, HM Revenue and Customs, and the Home Office - were involved in the review, which concluded that recognising multiple marriages conducted overseas was "the best possible" option. In Britain, bigamy is punishable by up to seven years in prison.

Islamic law permits men to have up to four wives at any one time - known as a harem - provided the husband spends equal amounts of time and money on each of them.

A DWP spokesman claimed that the number of people in polygamous marriages entering Britain had fallen since the 1988 Immigration Act, which "generally prevents a man from bringing a second or subsequent wife with him to this country if another woman is already living as his wife in the UK".

While a married man cannot obtain a spouse visa to bring a second wife into Britain, some multiple partners may be able to enter the country via other legal routes such as tourist visas, student visas or work permits.

In addition, officials have identified a potential loophole by which a man can divorce his wife under British law while continuing to live with her as his spouse under Islamic law, and obtain a spouse visa for a foreign woman who he can legally marry.

"Entry clearance may not be withheld from a second wife where the husband has divorced his previous wife and the divorce is thought to be one of convenience," an immigration rulebook advises. "This is so, even if the husband is still living with the previous wife and to issue the entry clearance would lead to the formation of a polygamous household."

Chris Grayling, the shadow work and pensions secretary, said that the decision was "completely unjustifiable".

"You are not allowed to have multiple marriages in the UK, so to have a situation where the benefits system is treating people in different ways is totally unacceptable and will serve to undermine confidence in the system.

"This sets a precedent that will lead to more demands for the culture of other countries to be reflected in UK law and the benefits system."

Mr Grayling also accused the Government of trying to keep the ruling quiet because the topic is so controversial.

The Telegraph