It was part of the opening session of talks at the annual gathering of leading industrialised nations being hosted at Lough Erne, in Co Fermanagh.
"Another £G8UK result: leaders agree to stamp out ransom payments to terrorists, calling on companies to follow lead." Mr Cameron announced via his official Number 10 Twitter feed.
Up to 70 million US dollars (£45 million) is estimated to have been paid to secure the release of Western captives in the last three years alone - an average 2.5 million US dollars (£1.59 million) per victim.
Much of that is believed to have ended up in the coffers of terror groups including al Qaida and its affiliates and the Taliban.
The UK outlaws such payments but other countries - including some within the group of leading industrialised nations - continue to meet the demands to the frustration of non-payers.
In the past, France, Italy and Japan have reportedly made payments to secure the release of their nationals but officials have been unwilling to specify any other countries which continue to do so.
The United States, Russia, Germany and Canada are the remaining G8 nations.
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