Sheikh Raed Salah, the leader of the Islamic Movement in Israel group, has been detained in London while on a speaking tour, the UK Home Office has confirmed.
Salah was detained late on Tuesday night for allegedly entering the country illegally, despite his organisation's insistence that he entered through formal and legal channels and had no knowledge of any travel ban.
Theresa May, Britain's interior minister, said in a statement to Al Jazeera that an investigation was under way into how Salah had been able to enter the country.
"We do not normally comment on individual cases but in this case I think it is important to do so.
"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."
But Salah's solicitor, Farooq Bajwa, quoted by the Guardian newspaper, said that his client had "no knowledge" of a travel ban and had made "no attempt" to conceal his identity when he entered Britain.
Islamic Movement spokesperson Sheikh Kamal Khatib said the arrest order was not yet clear, and the organisation had not yet spoken to Salah's lawyer.
"He was arrested on Tuesday night in London and is still in custody. We don't know yet if he will be deported but we are expecting to hear from his lawyer today," Khatib told the AFP news service.
The interior ministry said that there was a banning order in place, but would not confirm when or why it was imposed
Al Jazeera's Paul Brennan, reporting from London, said: "The organisers of his speaking tour here in the UK say it was a perfectly open arrangement.
"There was no secrecy about it. He didn't come in under a false name, he didn't arrive on a private jet. He came in through Heathrow airport, one of the main gateways into the UK.
"He arrived with his own passport, so if there was a banning order in place ... surely the border authorities would have stopped him then."
The Islamic Movement is tolerated in Israel, and unlike some Islamic groups is not banned in the country, although it is under constant surveillance by government forces.
Yvette Cooper, the shadow interior minister, criticised the government for its handling of the situation, saying that the coalition's stance of being tough on border controls had been "exposed as an incompetent sham".
Sarah Colborne, director of the Palestine Solidarity Campaign (PSC) - one of the groups organising Salah's speaking tour in Britain - told press: "The attempt to remove Sheikh Raed Salah from this country whilst he is taking legal action against those who have been defaming him is an obstruction of the course of justice.
"We have been organising a meeting on peace and justice in Jerusalem for weeks – with publicity widely available – to which Sheikh Raed Salah was one of the speakers. PSC also invited MPs to speak at the same meeting. At no stage did anyone contact us from the government or the police.
"Following rumours in the papers, Sheikh Raed Salah's legal team tried to verify if it was true that a travel ban had indeed been issued, and had no confirmation nor denial from any official source," she said.
"This shocking move by the British government will deeply damage British relations in the Middle East."
Jeremy Corbyn, a British MP, said that Salah should be allowed, as the leader of the Islamic Movement, to speak in the UK.
"It is crucial that spokespeople for the Palestinians - and the Sheik is the leader of the Islamic Movement in Israel - be allowed to come here to put forward the case for their rights," he said.
"The Israeli authorities continually make the claim that there is no Palestinian partner to negotiate peace with and yet so many of Palestine’s articulate spokespeople have been silenced."
He said that the government had not provided an adequate explanation regarding the travel ban.
'Astonished at arrest'
The PSC's Ruqayyah Collector told Al Jazeera that Salah spoke at a public event on Monday for over an hour, and encountered no problems.
Salah was due to speak on Wednesday night at the Houses of Parliament, alongside a group of MPs in an event "to highlight the plight of the Palestinian people in Jerusalem", that was widely advertised.
Ismail Patel, director of Friends of Al-Aqsa, was due to speak on the panel alongside Salah at the parliamentary event - and said the detention was the result of claims in British newspapers.
"I am astonished at his arrest," he said. "I am astonished that the home secretary can be influenced by the British media's accusations of [Salah] being an anti-Semite."
Wednesday night's panel was organised in conjunction with the Middle East Monitor, a UK-based research institute.
"We understand that Sheikh Raed will challenge the deportation order in court," said a spokesperson for the Middle East Monitor.
"It is not he who is losing his reputation; he has been arrested in Israel on countless occasions for defending Palestinian rights, and now the British government is acting in the same way.
"This will curtail freedom of speech in this country. At a time when the universal laws to prevent war criminals to enter this country unchallenged are being threatened, the double standards are clear.
"[The British government] judge Muslims on rumours and fabrications and allow Israeli war criminals to enter."
Salah, deemed to be widely respected as a leader among Israel's 1.3 million Arab residents, has had multiple run-ins with Israeli law enforcement, including being detained at the border with Jordan after allegedly striking an interrogator.
In 2010, he spent five months behind bars for spitting at an Israeli police officer, and he has been detained on a number of other occasions, although he denies most allegations and was acquitted of rioting charges from 2007.
He was also held after taking part in the Gaza-bound aid flotilla that Israeli naval commandos stormed on May 31, 2010, killing nine Turkish human rights activists.