Liaquat Ali Hazara, 36, is a campaigner for persecuted minorities in Pakistan, including the Hazaras, the Shia group to which he belongs.
Hundreds of Hazaras in Pakistan's restive Balochistan province have been killed in sectarian attacks by Sunni militants.
The Pakistan Human Rights Commission estimates that about 100,000 Hazaras have either fled to other parts of the country or left Pakistan in recent years.
"I am genuinely scared for my life, I have great concerns that I may disappear when I arrive at the airport," Hazara was quoted as saying by the BBC.
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The campaigner came to Britain in 2005 to study accountancy and began his political activities in 2009, founding the Hazara United Movement to draw attention to the plight of Hazaras in Pakistan.
More than 500 Hazaras have been killed in his home province of Balochistan since 2008, according to a recent Human Rights Watch report.
Shortly after starting his political campaigning, Hazara received death threats from Taliban militants and the al-Qaeda linked Lashkar-e-Jhangvi group, one of Pakistan's most violent Sunni Muslim militant organisations.
"They threatened to behead me if I return to Pakistan," he said.
The activist, who has been in detention since July of this year, applied for asylum in Britain in 2012 but his application was rejected.
A UK Home Office spokesman said it would not comment on individual cases, the report said.