What would the legal basis have been for stopping controversial Islamic cleric Yusuf Al-Qaradawi coming to the UK?
The cleric says he doesn't understand why there is controversy
The Home Office says that David Blunkett's powers to exclude Sheikh Al-Qaradawi do not come from any legislation, but are deemed a prerogative, although governed by case law.
Subject to a judicial review, the home secretary can exclude any individual who is seen to represent a threat to national security or a threat to public order.
The Home Office said the decision is always based on intelligence from the police and security services.
They say just 15 people were excluded last year in this way, but the most prominent example remains the controversial banning of the Nation of Islam leader Louis Farrakhan two years ago.
Court of Appeal
The banning of Mr Farrakhan, criticised for anti-Semitism, went all the way to the Court of Appeal, which upheld Mr Blunkett's decision.
But for somebody already in the country there is no similar arbitrary power, the Home Office said, and the home secretary would be obliged to follow the normal removal procedure, as is used against asylum seekers.
Senior human rights lawyer Geoffrey Bindman told BBC News Online the definition for who could be excluded remained someone whose "presence is not conducive to the public good".
"In this case what they are really concerned about is incitement and my argument here would be that up to now there isn't a case of incitement which could be presented to a court. Incitement to religious hatred is not unlawful.
'Incitement to violence'
"Incitement to violence would be difficult to establish. It is his claim that suicide bombers may be justified. He is not inciting them to do it."
But Mr Bindman said whatever the arguments about whether somebody was or was not a threat, the first part of judicial review, the Special Immigration Appeals Commission (SIAC) would be more likely to consider solely whether the right legal procedure had been followed.
Nation of Islam leader Louis Farrakhan was banned
"It would be very difficult to make out a case under the criminal law. The power to exclude is still available. I don't think there would be much chance of challenging a decision to exclude. I wouldn't see SIAC interfering.
In general, he said, the SIAC acts on procedural issues: "In the last resort they tend to leave it to the home secretary to make the decision. They are not likely to interfere if the home secretary is doing his best to weigh up the decision."