A proposal by the UN Secretary-General, Kofi Annan, to extend spousal benefits to the partners of gay UN officials has outraged representatives of the organisation's 51 Islamic nations, some of which outlaw homosexuality altogether. The Vatican's envoy to the UN has also expressed dismay, prompting fears among Mr Annan's aides of a joint Roman Catholic-Muslim crusade.

According to an internal UN bulletin issued in January, any "legally-recognised domestic partnership" will now qualify for the same UN employment benefits a conventional married couple would receive. The move has come as President George W Bush is proposing a constitutional amendment to prevent gay marriages in the United States. In the corridors of the UN, similar proposals are being mooted by conservative groups. Only nine of the UN's 185 members recognise same-sex partners.

They include Australia, Canada and France. Similar legislation is expected to go before Britain's Parliament this spring. In Morocco, by contrast, homosexual acts are punishable by between six months and three years in prison. Mr Annan's rule change means that a gay French or Australian UN employee should now be able to claim for his or her partner the same travel expenses, assistance with visas and pension plans as an ordinary husband or wife - or, for that matter, the multiple wives of diplomats from countries that permit polygamy.

Gay lobby groups claim that in the past relationships have broken up when UN employees re-located and same- sex partners were unable to follow. For nationals of countries where same-sex partnerships are not recognised, the new benefits would not apply. Traditionally liberal members of the UN general assembly have heartily approved Mr Annan's move. Canada's representative, Jerry Kramer, said that he "saluted" the new ruling. But the beginnings of a globalisation of gay marriage rights, seen by many UN members as a Western aberration, has caused near apoplexy among socially conservative colleagues.

Joseph Klee, the Vatican envoy to the UN, underlined last week that same-sex unions were contrary to Catholic teaching. "For us, marriage is a union between a man and a woman and is the foundation of the family," said Mr Klee. A spokesman for the Holy See told the Telegraph that if the new rule was implemented, the Vatican would attempt to block it. "So far we have made our position known informally," said the spokesman. "When the time comes for an official intervention we will make one."

Iran, where homosexuality is illegal, is leading Islamic objections, strongly supported by Egypt, Indonesia, Kuwait, Pakistan, Saudi Arabia and Syria. The Iranian UN representative, Alireza Tootoonchian, is demanding a vote on proposed spousal benefits for homosexuals at the UN General Assembly, where a majority would almost certainly vote against the idea.

As the Islamic world closes ranks, the European Union is attempting to come to Mr Annan's aid. "The European Union is satisfied with this practice and we see no reason why the secretary-general's prerogative in this area should now be contested," said a spokesman.

Privately, diplomats are more wary. "This is a hell of a hornets' nest," said one Western diplomat. "The Islamic countries are furious, although they are not sure they can do anything about it. The gay lobby is asking why there is such a fuss, when the UN recognises polygamous marriages and allows benefits to be divided between such wives.

The Americans, given President Bush's opposition to gay marriage, are keeping their heads down." Mr Annan has so far refused to take the issue before the General Assembly. His conservative opponents are promising to go to the UN Human Rights Commission in Geneva. "This one could run and run," said the diplomat.

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