Radical Islamic preacher Omar Bakri Mohammed can be kept out of the UK, Tory leader Michael Howard has said.
The Syrian-born cleric left London for Lebanon on Saturday but says he will return - unless the government says it does not want him back.
Deputy Prime Minister John Prescott on Tuesday said Bakri Mohammed was free to come in and out of the UK at present.
But Mr Howard said the home secretary had powers to exclude people who were "not conducive to the public good".
TRIO FACING SCRUTINY
Omar Bakri Mohammed, cleric for al-Muhajiroun. Its successor group the Saviour Sect being banned. Said he would not tell police if knew of UK bomb attack plans; supported Muslims who attacked British troops.
Abu Izzadeen, British-born, spokesman for al-Ghurabaa [the Strangers]. Would not condemn 7 July London bombings. Told BBC they would make people "wake up and smell the coffee".
Abu Uzair, former al-Muhajiroun member, told BBC the 11 September attacks were "magnificent". Said Muslims did not "live in peace" with the UK any more.
Mr Prescott, who is currently standing in for Tony Blair, said his message to Bakri Mohammed was: "Enjoy your holiday - make it a long one."
He added: "At the moment he has the right to come in and out. That is the circumstances at present and we have to change situations in this country by law.
"It's a democracy, not a dictatorship, for God's sake."
The Conservative leader said Mr Prescott was wrong about the rules.
Mr Howard told BBC News: "When I was home secretary, I excluded people from the country who hadn't been convicted of a crime here.
"So I believe that either that power should be exercised without delay or, if there is some reason which I haven't thought of why it can't be exercised, we should be told very clearly what that reason is."
The Home Office says it cannot comment on action against specific people but it is consulting on new grounds for deporting people or keeping them out of the UK.
A spokeswoman said: "We have published a list of unacceptable behaviours which would be grounds for excluding someone from the UK and will consider very carefully the exclusion of any individual who behaves unacceptably."
New laws will not be needed to introduce the new criteria for exclusions. Consultations began on Friday and will last for two weeks.
The new grounds could include fostering hate, glorifying terrorism and advocating violence as a way of furthering beliefs.
The spokeswoman stressed that the home secretary already had existing powers to exclude or deport foreigners whose presence is "not conducive to the public good".
This has generally been judged to mean posing a threat to: national security, public order or the UK's relations with another country.
Use of the powers can be tested in the courts.
Bakri Mohammed is thought to have dual Syrian and Lebanese nationality, but has indefinite leave to remain in the UK after gaining political asylum in the 1980s.
He ran the radical al-Muhajiroun group from Tottenham in north London until it was disbanded last year and is famous for praising the 9/11 hijackers as the "magnificent 19".
Bakri Mohammed on Tuesday denied he had fled the country and said he had never committed any crime in the UK or elsewhere.
He said he was staying with his mother for five or six weeks and if the British Government wanted to charge him with treason, he "would be the first one to return and challenge the allegation".
The Crown Prosecution Service is looking at whether remarks by three Muslim clerics, thought to include Bakri Mohammed, break any laws.
Director of Public Prosecutions Ken Macdonald QC says the talks have focussed on possible offences such as solicitation to murder and withholding information about terrorism.
But he adds: "There has been no discussion between us and the police about the offence of treason."