Immigrants with multiple wives will receive higher benefits payments in future as Government attempts to tackle the problem backfire.
Officials have admitted that polygamous households, of which there are an estimated 1,000 in Britain, could be handed more money under reforms to the welfare system.
Currently the state effectively recognises polygamous marriages by paying extra wives smaller amounts of income support in addition to the normal sum received by the husband and their first spouse.
When the streamlined Universal Credit regime is introduced next year, this policy will be scrapped.
But a House of Commons library paper admits that as an unintended consequence of this, any second or third wives in a household would be able to claim a full single person’s benefits in addition to the standard amount claimed by the husband and their first wife. So in total the polygamous family would receive more than they currently do.
The report states: “Treating second and subsequent partners in polygamous relationships as separate claimants could, in some situations, mean that polygamous households receive more under Universal Credit than they do under the current rules for means-tested benefits and tax credits.”
Baroness Flather, the first Asian woman to receive a peerage, said: “That is terrible. Why can’t they see it?”
She said the new benefits situation should prompt ministers to tackle the problem: “Why are they allowed to have more than one wife? We should prosecute one or two people for bigamy, that will sort it out.”
It is a criminal offence in England and Wales to be a bigamist, married to more than one person, but polygamous marriages are recognised in this country if the weddings took place overseas.
Men, usually Muslims from Pakistan, can bring one wife into Britain through the spouse visa route but further brides would have to arrive through a different immigration category. In 2007 MPs estimated that there were “fewer than 1,000 legally recognised polygamous marriages within the UK” but no detailed figures have been compiled.
Under the existing system, polygamous husbands and their first wives can claim the standard couples’ rate for income support of £105.95 a week while second and subsequent spouses living under the same roof receive an additional £38.45.
They can also claim housing benefit and council tax benefit if they reside together.
Ministers hoped to send a signal that polygamy would not be tolerated by removing these rules in the Welfare Reform Bill, which will introduce a single Universal Credit in 2013.
But it means second wives, living with their husbands and their first wives, will be able to claim as a single person, on top of the sum received by the couple.
Richard Fuller, the Conservative MP for Bedford and Kempston who raised the subject in Parliament earlier in July, said: “Most people will think it absurd that the benefit system currently recognises polygamy and ending this is one of the positive hopes from the introduction of Universal Credit. Only a few people are impacted by the change, but this sends a clear signal.
“Even though as a consequence some people may be able to claim more money as a result, the principle is worth it.”
A spokesman for the Department for Work and Pensions added: “Polygamy is illegal in this country and it would be wrong for the benefits system to legitimise these arrangements by recognising them in any way.”
Whitehall sources said they thought only a “very low” number of polygamous families would qualify for Universal Credit, levels of which have not been set.