British Muslim leader Sheikh Dr Zaki Badawi has said he has been refused entry to the US without explanation.
Dr Zaki Badawi joined other faith leaders in condemning the London bombings
The head of the Muslim College said he flew from London to New York to give a lecture at the Chautauqua Institution in New York but was turned back.
Dr Badawi said he was detained for six hours on Wednesday and that he was baffled and angry by the exclusion.
US Immigration and Customs Enforcement said it had information indicating Dr Badawi was "inadmissible".
Dr Badawi told BBC News he was angry that he was detained for six hours rather than being denied entry immediately.
"I really am mystified. I don't see any connection at all. It is completely illogical."
Dr Badawi appeared with fellow British faith leaders, including the Archbishop of Canterbury, on Sunday to deliver a joint statement condemning last week's "evil terrorist" attacks on London.
US customs spokeswoman Janet Rapaport said that when problems arose at JFK airport, Dr Badawi voluntarily withdrew his application to enter the country and returned home.
"We cannot disclose the information which led to the application being inadmissible because of privacy rules," she added.
Another spokeswoman for US customs on Washington, Leah Yoon, said when Dr Badawi was initially questioned his answers would not have been "in alignment" with his background check or documentation.
"He was questioned further and after a thorough interview he was deemed inadmissible," she said.
Dr Badawi, who is also a leader of the Council of Mosques and Imams, had been due to give a talk entitled The Law and Religion in Society.
Mike Sullivan, of the Chautauqua Institution, said all he knew was that Dr Badawi was back in London.
"We have no explanation as to why this happened," he said.
Salam Al-Marayati, executive director of the Muslim Public Affairs Council in America, said there was a need for moderates like Dr Badawi to come to the US "to counter the extremists".
"We need people like Cat Stevens who have committed themselves to fighting extremism," he said.
"People wonder why the moderates are not being heard. It is because they are being excluded."
Yusuf Islam, the British singer formerly known as Cat Stevens, was refused entry into the US and his plane diverted on national security grounds.
He most recently visited the US in 2003 - in the same year he was a guest of the Queen at a state banquet for US President George Bush.
And the theologian was the first prominent Muslim to criticise imams in the UK who did not teach in English.