Home Secretary Theresa May ordered an investigation today after a leading Palestinian activist accused of a history of "virulent anti-semitism" entered the UK despite being banned.
Sheikh Raed Salah, leader of the Islamic Movement in Israel, was arrested last night after entering Britain on Saturday to speak at a series of public events including a "roundtable discussion" in the Houses of Parliament.
Mrs May said: "I can confirm he was excluded and that he managed to enter the UK. He has now been detained and the UK Border Agency is now making arrangements to remove him.
"A full investigation is now taking place into how he was able to enter."
The arrest comes after Mrs May was questioned on Monday in the House of Commons by Tory MP Mike Freer about Mr Salah.
"The Home Secretary will be aware that Mr Raed Salah has been invited to speak in the palace precincts," he said.
"Given this man's history of virulent anti-semitism, will the Home Secretary ban him from entering the UK?"
Lawyers for Mr Salah, an Israeli citizen, strongly denied accusations of anti-semitism against their client and said they would fight the deportation order.
They said Mr Salah and his legal representatives had never been notified of an exclusion order and he had made no attempt to conceal his identity.
Farooq Bajwa, a solicitor acting for Mr Salah, said: "Had he been asked to attend a police station, he would have done so very happily.
"He made no attempt to conceal his identity, he has been attending public meetings and advertising those public meetings.
"This has not been a cloak-and-dagger visit. It was publicised well before his arrival that he would be coming."
He added: "The facts that we know are very clear - he has universally and consistently condemned all forms of racism including anti-semitism."
Tayab Ali, who is also acting for Mr Salah, said his client had been moved to an immigration detention centre after being held at Paddington Green police station last night.
"Mr Salah was in the UK to discuss the recent globally important developments in the Middle East with Parliamentarians and members of the British public," he said.
"To deport such an important and significant Palestinian activist is counter productive and acts against the principles of democracy that the British government states it wishes to export to that region."
Sarah Colborne, director of the Palestine Solidarity Campaign (PSC), insisted Mr Salah was from a legitimate political organisation and said he was opposed to all forms of racism including anti-semitism.
"Sheikh Raed Salah is the leader of the Islamic Movement in Israel, the largest movement for Palestinians in Israel," she said.
"This is a legitimate organisation which Israel has never moved to ban.
"Raed Saleh regularly speaks at venues across Israel where he has considerable support amongst the Palestinian citizens of Israel, who make up a fifth of the population.
"Sheikh Raed has been elected as mayor of his home town, Um al-Fahm, three times. He has never been convicted of anti-semitism in Israel.
"Before coming to Britain, he faced horrific allegations of anti-semitism, which he completely refuted.
"He has clarified his position of being opposed to all forms of racism, including anti-semitism, Islamophobia and racism against his own people, the Palestinians."
Shadow home secretary Yvette Cooper called for an urgent explanation as to how Mr Salah had come to enter the country unchallenged.
She said: "The Government's rhetoric of being tough on border controls has been exposed as an incompetent sham.
"The Home Secretary needs to urgently explain why an individual banned from this country was allowed to walk in and instead of being stopped at the border had to be pursued by the police instead.
"Only this week Theresa May said that coming to this country was a privilege and she would refuse entry to Britain of anyone that she deemed not conducive to the public good.
"Her words now ring very hollow indeed."
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