Five Iranians arrested by US troops in Baghdad last month were on a covert mission to influence Iraq's government, British officials have told the BBC.
The five men were senior intelligence officers "up to no good", an unnamed official told the Newsnight programme.
The arrests caused a diplomatic row when it became clear that the Iranians, who have since been released, had been invited by the Iraqi government.
Tehran has protested to the US, saying some of the men were diplomats.
The White House has suggested the arrests validated US claims of Iranian "meddling" in Iraq.
'No smoking gun'
A number of Iranians were arrested in the Iraqi capital on 21 December, when US forces raided a compound belonging to Abdul Aziz Hakim, leader of a powerful pro-Iranian Shia party.
"There were five senior officers in various Iranian intelligence organisations... these people have been collared, relatively speaking, up to no good"
Inside were Iranian officials and documents that, Newsnight's diplomatic editor Mark Urban says, caused consternation among coalition military commanders.
"There were five senior officers in various Iranian intelligence organisations... it was a very significant meeting... these people have been collared, relatively speaking, up to no good," an unnamed British official told the programme.
Officials told Newsnight the arrests produced highly important intelligence, but no "smoking gun" about weapons supplies or attacks on coalition forces.
They said that the arrested men were in Iraq to hold high-level meeting with representatives of several Iraqi Shia factions.
"There was discussion of whether the [Prime Minister Nouri] Maliki government would succeed, who should be in which ministerial jobs," one official told the programme.
"It was a very significant meeting... the fact of who some of the Iranians were is very important," the official said.
Following the arrests, the Americans quickly released several Iranian diplomats and three intelligence officers.
However, they continued for some time to hold two intelligence officers seized at the Hakim compound.
A US military spokesman in Baghdad told the New York Times newspaper that the raid had produced "specific intelligence from highly credible sources that linked individuals and locations with criminal activities".
The spokesman added that "some of that specific intelligence dealt explicitly with force protection issues including attacks on Multinational Forces".
Washington now wants the Iranians concerned to be banned from returning to Iraq.
The Iraqi government is reportedly considering such a step.
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