We should be told why a man accused of serious crime in the UK cannot be tried here – and just what MI5's role is in this case

For years, Abu Qatada has been in and out of prison in England. For years, successive governments have been trying to get rid of him. The question that is rarely asked, and never answered, is why has he not been tried in an English court? He is, after all, described by ministers – Theresa May, the home secretary, was at it again on Monday – as a serious threat to our national security. "A dangerous man, a suspected terrorist," she called him.

He was accused of a serious crime in his home country of Jordan, May added. But he has also been accused in England, not only of providing a "religious justification for acts of violence and terror", but also of being a "significant international terrorist, with extensive extremist contacts", engaging "in conduct which facilitates and give[s] encouragement to the commission, preparation and instigation of acts of terrorism", and "raising funds for terrorist groups".

MI5 has suggested, in evidence to the special immigration appeals commission (SIAC), that it has been difficult to prosecute Abu Qatada as he has been careful to disguise his support for terrorist attacks here and abroad, and that he is not "formally" a member of al-Qaida. Even so, unless the descriptions of Abu Qatada are no more than rhetoric, it is difficult to see how the plethora of anti-terrorist laws introduced over the past decade could not catch him.

Is it because the evidence against him was obtained by telephone taps? Such intercept evidence is inadmissable in British and Irish courts – but not in the US or Europe – on the argument that techniques developed by GCHQ and used by MI5 and the police must be protected from public scrutiny.

(It should be noted here, as an aside, that the inadmissability of wiretap evidence in any court could further extend the scope of the proposed justice and security bill due to be debated next week, introducing a whole new layer of "secret courts", where such evidence could be made admissible.)

Or is the reluctance to try Abu Qatada in England because for years after he was granted refugee status in 1994, MI5 believed that the individual frequently described as Osama bin Laden's "righthand man in Europe" did not really pose a serious threat to Britain's security, and that he was "all mouth"?

SIAC, in a key judgment five years ago, noted that Abu Qatada argued that MI5 "knew the sort of views he was expressing and took no steps to stop or warn him, to prosecute him or prevent his fundraising for groups which are regarded as terrorist groups".

Is this a reason why he is not prosecuted here? Would it be embarrassing if this was used in his defence in a criminal trial? We should be told why a man widely criticised for having made a mockery of British justice, to have cost taxpayers an estimated £3m in legal aid, prison, benefits and other costs, cannot be tried here.