Human rights and aid agencies have accused ministers of undermining the international plan to ban the use of cluster bombs.
The deadly munitions have killed and maimed innocent civilians in Lebanon, Iraq and Kosovo and many lie unexploded in former war zones for decades after conflicts have ended.
Groups including Amnesty International, Oxfam, Human Rights Watch, Handicap International and Landmine Action are pressing governments to agree to a comprehensive ban on cluster bombs.
Campaigners will this week campaign to persuade David Miliband, the Foreign Secretary, to drop the Government's backing for Britain's remaining cluster bombs and back a worldwide ban.
They will increase pressure on world leaders before up to 120 countries attend an international conference in Dublin next month charged with drawing up a treaty banning cluster bombs.
The Foreign Office said banning all varieties of the weapons would leave the armed forces with an "unacceptable capability gap".
A spokesman added: "This would have a direct impact on their ability to conduct combat missions in conflict zones."