The Home Secretary Charles Clarke used existing powers to exclude the so-called "Tottenham Ayatollah". The cleric left Britain six days ago on what he described as a holiday to see his mother in Beirut.
A Home Office spokesman said: "The Home Secretary has issued an order revoking Omar Bakri Mohammed's indefinite leave to remain and to exclude him from the UK on the grounds that his presence is not conducive to the public good."
The decision to bar Bakri indicated a change of position by the Government, which earlier this week said they were powerless to prevent him returning.
The Deputy Prime Minister John Prescott said on Tuesday: "I don't think he is welcome by many people in this country, is he?
"But at the moment he has the right to come in and out.
"It's a democracy, not a dictatorship, for God's sake."
Mr Prescott added: "I just say 'Enjoy your holiday, make it a long one'. "
It had been thought that immigration rules would have to be changed to keep him out.
The process to change the rules was launched by the Home Office last week and sources had indicated it would be complete by the time Bakri was due to return to London in four weeks' time.
It also emerged today that Bakri could be extradited from Lebanon to Syria.
A Lebanese Ministry of Foreign Affairs spokesman confirmed Bakri was still in custody today despite earlier reports of his release.
He also revealed that Syria had lodged an extradition request for Bakri, who caused outrage last week by saying he would not report Muslim bomb-plotters to the police.
"There is an official request from the Syrian authorities to surrender him to the security forces in Syria," said the spokesman.
"They say he is a Syrian and has been convicted in Syria for many crimes, and that they need him for those crimes."
He said he believed the activities for which Syria wished to see Bakri extradited were not terrorism-related but to do with "past events in Syria in the 1980s".
The extradition request would be considered by the government and would not require Bakri to remain in custody, the spokesman indicated.
The preacher's current detention was a routine procedure in Lebanon and he can be held for up to 72 hours, he said.
"The security forces are asking him about his permission to enter Lebanon, the circumstances of him entering Lebanon and leaving the UK,
"This is a very familiar procedure in Lebanon."
Bakri has seven children who were born in the UK and has relied on benefits for the past 19 years.
A Home Office spokesman said today's decision would not affect Bakri's family.
They would continue to receive their State benefits, he said, although those paid to Bakri would cease.
The preacher had also been due to have a £7,500 heart operation on the NHS later this year.
His £43.30-a-week disability allowance has been stopped for the duration of his stay abroad because he failed to tell the Department of Work and Pensions that he was leaving the country.
Bakri's spokesman, Anjem Choudray, described the move as "completely outrageous" and a "failure" of the principle of free speech.
"I think it is completely outrageous that the Government can just exclude someone on the basis that they disagree with his views," he told Sky News.
"He has never been charged with a criminal offence in this country. He has been living here for 20 years - he has a wife, children and grandchildren here.
"He has been a great asset for the Muslim community here. It is going to be a great loss for the British public and the Muslim community, I believe, and I think that this is indicative of the oppressive nature of the Blair regime."
Mr Choudray added: "He came here as an asylum seeker thinking that this country had freedom of speech and freedom of religion and then when he practised his freedom, suddenly laws were introduced and backdated and the Government is threatening deportation - obviously now they have banned him.
"It seems to me this is a failure of the freedoms and values that you espouse. The Muslims around the world will see this as a great victory for Islam and see Islam as a superior ideology."
Inayat Bunglawala, from the Muslim Council of Britain, said: "Omar Bakri is unlikely to be missed by the vast majority of British Muslims.
"He is someone who for 20 years was given shelter by this country and he has spent almost all that time vilifying this country and its values.
"With his often very offensive remarks he has contributed towards the demonisation of British Muslims."
Bakri, who has been investigated by police over his allegedly inflammatory language but never charged, was expelled from Saudi Arabia as an extremist and arrived in Britain in 1986.
Last week, the Prime Minister announced that the radical al-Muhajiroun group, which Bakri founded, and its successors, were to be added to the list of banned terrorist groups.
- More about:
- Labour Party