Iranian officials had previously said the April 12 blast, during an evening prayer sermon by a prominent local cleric, was caused by explosives left over from an exhibition commemorating the 1980-88 Iran-Iraq war.

Judiciary spokesman Ali-Reza Jamshidi told state television the terrorists behind the bombing were agents of the U.S. and British governments in Iran.

"The relationship of those who planted the bombs in Shiraz with the U.S. and Britain was identified and they were being financially supported and in fact they acted as foreign agents in Iran," he said.

"In view of the documents obtained the judiciary in cooperation with the government and the Foreign Ministry will file lawsuits with international authorities against their supporters, who on the one hand claim to fight terrorists and on the other hand provide them with equipment," he said.

He was clearly referring to Britain and the United States, but did not give details on how Tehran would take legal action against them.

Iran has in the past accused the two countries of trying to destabilise the Islamic Republic by supporting rebels, mainly those in sensitive border areas.

Tehran's accusations over the mosque blast echo allegations U.S. officials have made about Iranian support for militias in Iraq that have fought U.S. and U.S.-backed government forces there, accusations Tehran denies.

Iran and the United States are also at odds over Tehran's nuclear programme, which Washington suspects is aimed at making bombs. Iran says its programme is for producing electricity.

Security is normally tight in Shi'ite Muslim Iran and bomb attacks have been rare in recent years. Several people were killed in 2005 and 2006 in blasts in a southwestern province with a large Sunni Arab population.

Shiraz is a southern city with more than one million inhabitants and is a popular tourist destination.