Controversial right-wing Dutch politician Geert Wilders has been stopped from entering the UK on the grounds that he could be a threat to public order.
Labour peer Lord Ahmed has said his presence would pose a risk because his views about the Koran - he has called it a "fascist book" - generate so much anger among Muslims.
Aside from Mr Wilders, 270 people have been prevented from entering the UK since July 2005.
The reason? Their presence was "not conducive to the public good".
In other words, in the eyes of the home secretary, who has the final say, they pose a threat to national security, public order or the safety of UK citizens.
Such a power does not come from any legislation, but from case law, and can be challenged through a judicial review, in which the person concerned must present evidence that they are not, in fact, a threat.
Those excluded in recent years include religious extremists, neo-Nazis, animal rights activists, rap artists and lifestyle gurus.
In October 2008, Home Secretary Jacqui Smith announced that a full list of those banned was to be published and shared with other countries.
Until it is, the general public has no way of knowing who they are unless they draw attention to their situation themselves.
It is known that about 80 of those barred have been so-called "preachers of hate".
Among them is Lebanon-based cleric Omar Bakri Mohammed, the former head of the now-disbanded group al-Muhajiroun, who gained notoriety for praising the 9/11 hijackers as the "magnificent 19".
In 2005, the then home secretary Charles Clarke used his powers to revoke Mr Mohammed's right to remain as an asylum seeker and banned him from Britain altogether.
Other preachers to have raised the ire of the authorities include Nation of Islam leader Louis Farrakhan, from the US, and Yusuf al-Qaradawi, who lives in Qatar.
In February 2008, Muslim cleric Mr al-Qaradawi was refused a visa because the Home Office said the UK would not tolerate the presence of those who sought to justify acts of terrorist violence.
The Egyptian-born cleric was described at the time as "dangerous and divisive" by Conservative leader David Cameron.
A long-term ban against Louis Farrakhan had been in place for a number of years when he failed in 2002 to have it overturned by going through the courts.
The then home secretary David Blunkett said that a visit from the Nation of Islam leader, who has made anti-Semitic and racially controversial statements, would threaten public order.
But the exclusions have not just been directed at religious figures - in 2004, Mr Blunkett wrote to US animal rights activist Dr Jerry Vlasak to say he would not be allowed into the UK.
He was barred after reportedly saying that millions of animal lives could be saved if a handful of vivisectionists were killed.
Martha Stewart and Snoop Dogg are also among those to have been denied entry to the UK from the US - for them, the justification was that they had convictions.
Snoop was previously cautioned on suspicion of violent disorder and affray, while Stewart was jailed in the US for lying to investigators about a share sale.
Another American rapper Busta Rhymes - real name Trevor George Smith Jr - was detained at London City Airport last September.
He was prevented from entering the UK due to "unresolved convictions" in the US, but a High Court judge then agreed he could stay in order not to disappoint fans who were expecting to see him in concert.
The UK Border Agency will not comment specifically on individuals, but it has said it opposes the entry of anyone convicted of "serious criminal offences abroad".