A militant alleged to have been part of the cell that murdered former Pakistani PM Benazir Bhutto has appeared in a Taliban video denying his involvement.

Ikramullah is believed to have been a back-up suicide bomber, who was meant to detonate his explosive vest if the first attacker did not succeed.

But officials say he walked away after the other bomber blew himself up, killing Ms Bhutto and at least 20 others at rally in Rawalpindi in 2007.

A senior Bhutto aide said he was lying.

In his first public statement on the case, Ikramullah appears in a video produced by a splinter group of the Pakistani Taliban which was obtained by the BBC. It is believed to have been filmed in eastern Afghanistan, where the militants are based.

Described as a "senior figure" in his group, Ikramullah repeatedly states in the video he was neither "involved" nor "aware of" the plot to kill Bhutto. He is on a Pakistani list of most-wanted terror suspects, and has been named in court as the second suicide bomber.

Senator Rehman Malik, a former interior minister who was a close friend of Bhutto's, told the BBC that he believed Ikramullah was "totally lying", and that other suspects had named him in court as the second bomber.

A source with knowledge of Pakistani militant groups told the BBC that until recently Ikramullah was openly and proudly claiming his involvement. But last year he was attacked by other rival Islamists in Afghanistan, and his family received threats from the Pakistani security services.

As a result, it is believed, he was advised by his group's leaders to make a video denying his involvement. The source told the BBC: "The whole of the Pakistani Taliban, and even young children from the tribal areas know he was involved."

Bhutto was elected as prime minister in 1988 and 1993. After a period in exile she returned to Pakistan in 2007 to campaign for elections.

She survived an assassination attempt in October 2007 when suicide bombers targeted a parade welcoming her at Karachi airport. More than 150 people died.

Bhutto was killed two months later at a rally in Rawalpindi. Five alleged militants charged with involvement in the plot were last year acquitted, but remain in detention pending an appeal.

The leader of the Pakistani Taliban at the time, Baitullah Mehsud - who died in US strike in 2009 - denied that the group was responsible. However, many have speculated that this was prompted by the public backlash the attack received.

Pakistani intelligence officials said they had intercepted a phone conversation in which Baitullah Mehsud was told by an unnamed cleric that the attackers were "our men" and included Ikramullah, who was then about 16 years old.

Earlier this year, a book published by the Pakistani Taliban's main faction on the history of the group acknowledged that despite earlier denials they had indeed carried out the attack, and again named Ikramullah as the second suicide bomber.

The book, From British Raj to American Imperialism, was written by Mufti Noor Wali Mehsud, who was named over the weekend as the new head of the main Pakistani Taliban. The group's previous leader was killed earlier this month in a drone strike.

According to the book, Bhutto was targeted by the militants because she was planning "to target the mujahideen" and create a government sympathetic to "American interests."

The video in which Ikramullah features is produced by a splinter faction of the Pakistani Taliban rather than the main group. No mention of the book is made in the video. However, the commander of the splinter group, Shehryar, says attempts "to link Ikramullah to the killing of Benazir is the work of the infidel media and apostates".

Former Interior Minister Rehman Malik told the BBC that Ikramullah was "the only living person" with inside information on the Bhutto case, as the majority of other militants linked to it have since been killed. Mr Malik added that Ikramullah's denial could be motivated by fear that he might eventually be detained in Afghanistan and handed over to Pakistan.

Alternatively, he suggested, the denial could be a way to challenge the authority of the new leader of the main Pakistani Taliban group. Mr Malik is calling for Ikramullah to be captured and put on trial.

Shehryar also claims in the video that the Pakistani Taliban were not involved in the attack, and blames it on the country's then-ruler Pervez Musharraf and the intelligence services.

Mr Musharraf has been declared a "proclaimed offender" after being charged with "abetment and conspiracy to murder" in relation to Bhutto's death, but not appearing in court.

He is currently in self-imposed exile in Dubai. He has always denied any involvement in Bhutto's death or having deliberately not provided her with adequate security.

Many commentators believe the Pakistani Taliban were involved but also that the full facts surrounding the attack have been "covered up."

A 2010 report by the United Nations describes the initial police investigation as a "whitewash".