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By Peter Graff

LONDON, March 20 (Reuters) - Britain became on Tuesday the biggest military power to ban "dumb" cluster bombs without self-destruct devices, but landmine charities said the move was not enough to protect civilians.

Cluster weapons are designed to kill large numbers of enemy troops by scattering hundreds of tiny bomblets that spread deadly shrapnel over an area.

Campaigners say the bombs often fail to explode on impact, and unexploded ones can kill civilians years after a war ends.

Britain and the United States used thousands of cluster bombs during the invasion of Iraq in 2003 and Israel fired them widely into southern Lebanon last year during fighting with Hezbollah guerrillas.

Britain's Ministry of Defence said it would now destroy its stockpile of "dumb" cluster munitions -- its term for bombs that do not self-destruct -- and use only weapons with self-destruct mechanisms limiting the number of unexploded bombs left behind.

"It is our duty to make sure our forces have the equipment they need to do the job we ask of them. At the same time, we should strive to reduce civilian casualties to the minimum," Defence Secretary Des Browne said in a statement.

"Military commanders are first to point out that modern conflicts are in large part about winning hearts and minds."

But Simon Conway, director of Landmine Action UK, said the self-destruct mechanisms on the newer British bombs, identical to ones used by Israelis during a war in Lebanon last year, were not reliable enough to make them safe.

"These are littered all over southern Lebanon because they failed and the self-destruct mechanisms failed," he said.

In some cases the new bombs have actually proven harder to clear than the old ones because the self-destruct mechanism carries an extra detonator, making them even more dangerous.

Austria and Norway have banned all unguided cluster weapons, not just those without self-destruct mechanisms. Still, he said, it was a sign of progress that Britain was acknowledging that civilian casualties were a problem.

"It's a good step. What they need to do is acknowledge that sticking a self-destruct mechanism onto a munition isn't enough," he said. "In our view, 'smart' means precision-guided, and attaching self-destruct mechanisms to an unguided area effect weapon does not make it 'smart'."

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