In a resignation statement that provoked stand-up conflict between the MP and Betty Boothroyd, the Speaker, Mr Mates said the SFO had not only created a process of trial by media leak, but had alleged a conspiracy to bribe Mr Justice Tucker, the trial judge - and had capped that with a claim that the judge himself was suspected of conspiring to pervert the course of justice.
Calling for an independent inquiry, he said: 'This appears to have been a deliberate attempt to destabilise the defence.'
He added that he had verbatim transcripts of court proceedings to substantiate everything he had said, and that he had overcome the initial doubts of Sir Nicholas Lyell, the Attorney General, to convince him that the SFO had levelled the 'bizarre' charge against the judge.
'He felt sure I had got it wrong, and the alleged conspiracy was against the judge, rather than involving him,' Mr Mates said. 'He now knows I was right.'
However, within minutes of the statement's delivery, that was disputed. The Prime Minister's office said: 'All the allegations which have previously been put to the Attorney General have been the subject of careful inquiry and fully rebutted.'
Sir Nicholas did not specify who had carried out those inquiries. He told Channel Four News that he had had each of Mr Mates's allegations 'looked into in the appropriate quarter'.
Ruling out any question of an inquiry, the Government's senior law officer added: 'Either I discovered that there was no foundation for them, or that there was some basis of fact but that there was a reasonable explanation . . . . I found nothing to suggest that the Serious Fraud Office had done the things which Michael Mates brought to my attention.'
Asked by the Independent whether he thought Sir Nicholas had asked the SFO to investigate itself, Mr Mates grinned, saying: 'You would say that; I couldn't possibly comment.' However, he did say that an inquiry could settle the dispute between himself and Sir Nicholas.
George Staple, director of the SFO, said: 'If anybody has any evidence of any leaks, and they want to bring them to me, then I will treat them with the utmost seriousness and investigate. At the moment, I've got no evidence.'
The Commons Home Affairs Select Committee will today consider the possibility of asking for a special hearing with Mr Staple to put its own questions to him about the allegations of dirty tricks. Mr Staple said last night, however: 'I'm not aware of any dirty tricks we've engaged in.'
Meanwhile, Mr Mates sought to justify his controversial intervention - which led to his loss of office - on the issue of principle. He had written to Sir Nicholas because of 'questions of apparent injustice' - rather than the matter of Mr Nadir's guilt or innocence.
But in some of the most breath- taking scenes seen in the Commons for many years those questions drove Mr Mates to the cliff-edge of parliamentary defiance.
Miss Boothroyd initially cautioned Mr Mates against a breach of House sub judice rules, but he ploughed on regardless, repeatedly interrupted by the Speaker to the point at which she ordered the defiant MP 'to resume his seat'.
In normal circumstances that would have ended the battle of wills - but Mr Mates protested that if he was not allowed to point out problems with the system of justice, 'then what is the point of our being here?'
Miss Boothroyd angrily snapped back: 'It is after the trial that the honourable gentleman must give this information.' Mr Mates persisted, reading from a prepared text.
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