Iran offered to halt attacks on British soldiers deployed in Iraq in return for a secret pact that would enable it to continue its nuclear programme, a senior British diplomat has said.

Sir John Sawers, the British ambassador to the United Nations, revealed that Iranian officials openly acknowledged complicity in attacks that killed scores of British soldiers in southern Iraq.

In private talks in hotels around Europe, the unnamed Iranians floated a grand bargain that would have derailed efforts to impose sanctions on Iran to stop its covert nuclear programme.

"There were various Iranians who would come to London and suggest we had tea in some hotel or other," Sir John tells the final part of a BBC documentary on the Islamic Republic's relations with the West, to be broadcast on Saturday. "They'd do the same in Paris, they'd do the same in Berlin, and then we'd compare notes among the three of us.

"The Iranians wanted to be able to strike a deal whereby they stopped killing our forces in Iraq in return for them being allowed to carry on with their nuclear programme: 'We stop killing you in Iraq, stop undermining the political process there, you allow us to carry on with our nuclear programme without let or hindrance'."

Iran supplied arms, training and strategic direction to Shia Muslim militias that were battling British forces for control of Basra and other southern cities. Iran supplied the militias with a deadly bomb, the shaped-charge explosive device, that required precision engineering at its military factories.

As Iran marks the 30th anniversary of the revolution that turned out the Shah and installed a cleric-led regime, senior figures have openly discussed a series of secret deals with West. Iran had used its involvement in hostage taking during the Lebanese war to break its isolation in the 1980s.

Iran also gave the US intelligence on Taliban and al-Qaeda locations in Afghanistan on several occasions and offered to join a coalition against Saddam Hussein in Iraq.

A former Iranian president, Hashemi Rafsanjani, told the programme that he had personally authorised Iran's role in ordering the release of Western hostages, including Terry Waite and John McCarthy, after a plea from President George Bush.