Mujeeb ur Rehman Bhutto's gang were behind a high-profile kidnapping in Karachi in 2004 and he then took a 56,000 pounds ransom payment in Manchester, BBC reported
In 2005, Bhutto, of Leeds, admitted being the gang's "boss" and was jailed for seven years by a UK court.
UK Independence Party (UKIP), which wants Britain to leave the European Union, said Bhutto, 35, had "recently" resigned his party membership.
A party spokesman said: "When we recently became aware of possible issues relating to his past and raised the matter with him, he resigned his membership."
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Bhutto told BBC's Newsnight he had admitted the charges against him in 2005 rather than risk being sent back to Pakistan and hanged.
"The evidence which was brought against me was from Pakistan. The allegation was simply because of political rivalry," he said.