The MoD last year described the Hydra CRV-7 system, which delivers a number of bomblets from a helicopter-mounted rocket pod, as a cluster weapon.

Later in the year, Margaret Beckett, then foreign secretary, said the government did not consider the weapon fell within the term "dumb", because virtually all the bomblets exploded on impact. Des Browne, the defence secretary, announced earlier this year he had banned the use by Britain's armed forces of "dumb" cluster bombs. He said Britain would deploy only cluster bombs which had a self-destruct mechanism.

In July, Bob Ainsworth, the armed forces minister, told MPs the CRV-7, equipped with "multi-purpose submunitions" fired from Apache helicopters, did not after all fall within the government's "understanding of a cluster munition". Cluster munitions release smaller "bomblets" when fired, posing a danger to civilians during and long after a conflict. Unexploded "bomblets" maim and kill civilians in Afghanistan, Iraq and Lebanon.

Mr Ainsworth said in July CRV-7 bombs were not now cluster weapons because of "direct fire capabilities" and because they had "too few submunitions".

However, the MoD admitted trials in the US had revealed a 6% failure rate. Though the MoD says the CRV-7 bomb has nine submunitions, critics say today it is a 19-rocket pod, with 171 submunitions in all.

The reclassification is attacked today by Oxfam, Amnesty International, Human Rights Watch and Landmine Action, who also say that Britain has been the world's third largest user of lethal cluster bombs over the last 10 years. "Ten years after it championed a treaty banning landmines, the UK has a chance to do the same with cluster bombs - but instead it is spinning a cluster bomb con," said Simon Conway, Director of Landmine Action.

Tom Porteous, London director of Human Rights Watch, said: "Our investigations in Kosovo, Afghanistan, Iraq and Lebanon have all shown that cluster munitions, no matter how sophisticated, do not work as advertised, and instead get used in ways that violate international humanitarian law."

In February 2007, Britain joined 46 other countries in calling for a worldwide ban on cluster bombs. This initiative, the Oslo Process, is expected to lead to a treaty next year, the humanitarian groups say.

Anna MacDonald of Oxfam said: "Current UK policy on cluster bombs makes no sense. They say they want an international treaty but they also want to keep using cluster bombs well known to kill and injure civilians."

Britain also has in its armoury artillery-delivered M85 cluster bomblets which are supposed to self-destruct if they do not explode on impact. A Commons foreign affairs committee report estimated that the weapons had a 10% failure rate.

"Despite being the third biggest user of cluster bombs in the world over the last 10 years, the government hasn't made any efforts to asses the harm these weapons cause to civilians," said Oliver Sprague of Amnesty International.

In a statement last night, the MoD said: "The UK is committed to improving reliability of all munitions, including cluster munitions, with the aim of achieving lower failure rates and leaving less unexploded ordnance. There is no internationally recognised definition for cluster munitions." The statement added that an MoD assessment concluded "direct fire munitions" should not be classed as cluster weapons and that was the view shared by the Red Cross; "direct fire" normally means the gunner can see the intended target.