But mystery surrounded the alleged spying device, as senior diplomats in the Iranian embassy said they did not know when or where in the building the object had been found; they claimed they had had no knowledge of the device until President Ali Akbar Hashemi Rafsanjani mentioned it in a speech on Wednesday.
In Tehran, the Iranian government said the device had been found in a wall two weeks ago. Mr Rafsanjani said embassy staff had discovered 'the hidden microphones installed by British MI6'.
In London, Mohammad Safai, the embassy's deputy head of mission, told the Independent: 'I'm not aware when and how it was discovered. I believe in a reconstruction or repairing of the wall. We have got a different group of Iranian workmen checking things.' Asked if he believed the device had been planted during the refurbishment of the embassy before its reopening last December, he said it was 'one possibility', adding: 'It's very difficult to make a judgement when I have very little information other than what Mr Rafsanjani and Mr Vaezi have said.'
In Tehran, the Deputy Foreign Minister, Mahmoud Vaezi, said the bug was not disclosed two weeks ago because 'due to intelligence and operational considerations we needed to investigate the matter further'.
'The device weighs about half a kilo,' he said. 'It is composed of several parts and has a battery inside. It acts as a transmitter.'
The British charge d'affaires, Jeffrey James, was handed a strong protest during the 45-minute meeting in Tehran. Mr Vaezi said: 'We demanded that the British government explain this action. The British government's explanation will determine our next steps.'
The British Foreign Office was markedly tight-lipped, saying it had 'no comment' to make. Asked why not, an official said: 'No comment.'
Two weeks ago, the British government summoned the Iranian charge to demand assurances that Iran sever its links with the Provisional IRA. British officials said they had first-hand intelligence of an Iran-IRA plot.
Mr Rafsanjani said on Wednesday that Britain had invented the plot charge to seek to avert a scandal over the bugging. 'What the British started up about Iran's co-operation with the IRA was an out-and-out lie. They provided no evidence,' the President said. 'Exactly on the same day, we had discovered a secret microphone that they had installed. This could create a scandal for them. So they raised a hue and cry to pre-empt this.'
There was some confusion as to what day Mr Rafsanjani was referring to. The Foreign Office summoned the Iranian charge on 28 April; but an article about the Iran- IRA links had already appeared on 25 April in a Greek newspaper. There was speculation the article was a deliberate leak from British intelligence.
The Iranian embassy in Prince's Gate, Kensington, was reopened in December, 13 years after it was gutted during a hostage-taking that ended with its storming by an SAS team. The complete refurbishment of the embassy cost pounds 4m, part of it paid for by Britain.
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