At the end of a two-day meeting in Oslo, Britain shifted dramatically its position and supported a declaration committing governments to "conclude by 2008 a legally binding international instrument" to ban cluster bombs.

The munitions are notorious because they continue to kill and maim long after conflicts have ended, and particularly harm children, who are attracted by the bright colours and small size of unexploded bomblets. Thousands of the munitions are still littered across Lebanon after an estimated 4 million were dropped by Israel during the war last summer. As many as 40 per cent of the munitions can fail to explode on impact.

Activists said Britain's sudden switch in position would create an unstoppable impetus towards a treaty, despite key cluster bomb producing states such as the US, Israel, China and Russia remaining outside the process. "The fact that the UK weighed in meant that the whole Oslo process will succeed - we will see a treaty," said Simon Conway, the director of Landmine Action, which was behind a similar successful process to ban anti-personnel mines.

The declaration adopted yesterday said a treaty would "prohibit the use, production, transfer and stockpiling of those cluster munitions that cause unacceptable harm to civilians." As well as setting a date for conclusion of a treaty, the non-binding text lays down a timetable for negotiations.

Activists had feared that Britain, which uses the munitions, went to the conference to water down the language in the draft declaration. As of Thursday night, according to conference delegates, Britain would only agree to "consider" a treaty, rather than commit to the conclusion of a pact.

Yet that changed yesterday morning. Mr Conway said: "In a dramatic last minute statement, the UK, which has used so many cluster bombs in the past, showed real leadership and agreed to join a fast track process to negotiate a ban on cluster bombs that cause unacceptable harm to civilians. The UK should now commit to dispose of their stockpile of "dumb cluster munitions as soon as possible".

The Blair government has been split over the issue, with the Ministry of Defence opposed to an outright ban, while the International Development secretary, Hilary Benn, argued that the weapons should be outlawed. The foreign office was said to be "weaving in the middle".

A Foreign Office spokesman said this week that while the British delegation was going to Oslo with an "open mind", Britain hoped Oslo would "complement the process already agreed" at the UN-sponsored convention on conventional weapons in Geneva last November which included all the weapons states, but which failed to declare a ban on cluster bombs as a final objective. The failure of the November CCW meeting prompted the Norwegian government to convene the Oslo conference.

The Norwegian Foreign Minister, Jonas Gahr Stoere, said the conference surpassed expectations. But the outcome is unlikely to please UK allies such as the United States, which boycotted the successful attempt to negotiate a landmine ban, culminating in a 1997 treaty now backed by 153 countries.

Although the current declaration is not legally binding, "this will put immense pressure on the US", said Mr Conway. "Cluster bombs have now been stigmatised as a weapon."

Thomas Nash, a spokesman for the Cluster Munition Coalition, said he was encouraged by the small number of states opting out of the final declaration, after cluster bomb states such as France, Italy, the Netherlands and Egypt joined in. Only Japan, Poland and Romania did not approve the final text, which was approved by 46 out of the 49 states in attendance. Campaigners said they would continue to monitor the implementation of yesterday's declaration which also urged countries to take steps at a national level before the treaty takes effect.

Oxfam is calling on Britain to follow the lead of other cluster-bomb states and end its opposition to declaring a moratorium on the use of the munitions which are packed by the hundreds into artillery shells, bombs or missiles which scatter them over vast areas.

Austria, a stockpiler of cluster bombs, declared a moratorium this week, after Norway halted their deployment in June last year. Belgium became the first country to ban cluster bombs, in February 2006.