Gordon Brown has negotiated a loophole for Britain to continue using cluster bombs, despite his declaration of a full ban.

The prime minister appeared to reinforce his humanitarian credentials when he dramatically overruled the Ministry of Defence (MoD) in talks at a 109-nation conference in Dublin on Wednesday.

While Brown announced support for “a ban on all cluster bombs, including those currently in service by the UK”, the government quietly excluded new anti-tank cluster shells that are not yet in service.

Britain will now press ahead with an £83m contract to buy a new generation of the munitions, signed last November with GIWS, a German manufacturer.

Brown’s intervention in Dublin does mean an end to Britain’s two existing “smart” cluster munitions. The M85 artillery shell, which splits up into 49 bomblets and was last used in Iraq, will be taken out of service immediately. The M73 rocket, fired from the army’s Apache helicopters, contains nine bomblets and is deployed in Afghanistan. It will be phased out over eight years. By then the new ballistic sensor fused munition shell will be in service. The shell splits into two bomblets that descend on small parachutes, which make them particularly attractive to children if they do not detonate.

Although the MoD has previously described the shells as “cluster munitions”, it now maintains they do not fall into the convention’s final definition of what constitutes a cluster weapon.

Britain, France, Germany and Sweden, which all use or manufacture similar weapons, pushed through amendments to the treaty to exclude them because of their size and ability to self-destruct.

Cluster weapons, whether dropped by aircraft or fired from artillery, split up into bomblets, which can fail to explode.

The MoD initially sought to exclude all Britain’s existing cluster munitions on the grounds they are “smart” munitions that self-destruct if they do not hit their target. But opponents say there is a 30% risk that even then they will not explode. If they do not, their self-destruct mechanism only makes them more dangerous to civilians.

Nick Harvey, the Liberal Democrat defence spokesman, said their exemption at Britain’s request “completely undermines the moral argument the prime minister says he is making”.

Their exclusion will be reassessed in five years.

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