Illegal British made cluster bombs have been found in Yemen, according to Amnesty International, adding confirmation that the banned weapons are used by Saudi-led forces.

The human rights groups has called for an investigation into whether UK nationals have broken the law after Britain signed an international convention in 2008 that banned the use of cluster bombs.

Evidence of British and American made cluster bombs was found to have been used by the coalition supporting the Yemeni government, led by Saudi Arabia and the United Arab Emirates. Neither have signed the 2008 convention, nor has the United States, or Brazil but under the law British nationals are banned from using the bombs or assisting in their deployment.

One type of British made bomb discovered is designed to be dropped by a Tornado jet, dozens of which have been sold to Saudi Arabia from Britain in recent decades. The planes are still services by British mechanics, leading to the call for an investigation by Amnesty.

"The discovery of the cluster bomb is the first clear evidence that, as long suspected, members of the Saudi Arabia-led military coalition have used British cluster munitions in their highly controversial attacks in Yemen," the charity said in a statement.

"Given that the UK is known to have several hundred specialist support staff working closely with the Royal Saudi Air Force, Amnesty is warning that any involvement of UK personnel would constitute a clear breach of the UK's legal responsibility under the Convention on Cluster Munitions."

The bomb is designed to spread small pieces over a large area so its target is indiscriminate and a particular danger to civilians.

Amnesty International have campaigned against Britain's arms sales to Saudi Arabia. He said it would be an "absolute scandal" if British personnel were found to be involved.

But the foreign secretary Philip Hammond strongly defended the sale of arms to Saudi Arabia.

"The UK is not a member of the Saudi-led coalition," said a spokesman. "British personnel are not involved in carrying out strikes, directing or conducting operations in Yemen or selecting targets and are not involved in the Saudi targeting decision-making process."

However the Saudi foreign minister has previously said British military advisers assist in Saudi-led bombing raids in Yemen and know the list of targets.

The bombing campaign has been heavily criticised for hitting civilian targets across Yemen and the British government is facing particular criticism as a major supplier of weapons to Saudi Arabia, which is a major ally in the Middle East.