The US president told the UN last month member countries should support a Costa Rican proposal to ban both reproductive and therapeutic cloning.

But the Royal Society urged countries to back a second proposal from Belgium at the UN vote on 20 and 21 October.

The proposal would allow countries to make their own decision on therapeutic cloning but bans reproductive cloning.

Therapeutic cloning has been legal in Britain since 2002.

It involves cloning embryos and harvesting stem cells from them.

The embryos are destroyed before they are 14 days old and never allowed to develop beyond a cluster of cells the size of a pinhead.

Reproductive cloning - the cloning of human embryos with the intention of creating a baby - is still strictly banned in the UK.

Lord May of Oxford, president of the Royal Society, the UK's national academy of science, said: "The US should be allowed to decide whether therapeutic cloning should be outlawed within its borders.

"But other countries, including the UK, have now passed legislation to allow carefully regulated therapeutic cloning while introducing a ban on reproductive cloning.

He warned if the UN could not reach a workable resolution the risk was that maverick scientists would attempt reproductive cloning in countries that have not passed laws against it.

Ban

Lord May said: "The alternative Belgian proposal, which is backed by the UK government, seeks to ban human reproductive cloning worldwide but to allow individual countries to make their own decision on therapeutic cloning.

"If this proposal was successful, the United States and others would still be free to ban all human cloning but countries that see the promise offered by therapeutic cloning can still carry out research."

Lord May added: "The US government's approach at the UN appears more designed to influence domestic legislation, where attempts to introduce a total ban have so far failed, at the expense of a workable international ban on reproductive cloning.

"Indeed, the US has not yet outlawed reproductive cloning itself."

Last year, the UN voted by a slim margin to postpone the decision, despite powerful lobbying by the US.

Earlier this year, scientists from the University of Newcastle were granted a licence to clone human embryos in the UK for medical research.

Stem cells from early embryos could potentially be used to provide new treatments for incurable diseases such as Alzheimer's, diabetes and Parkinson's.