Rashid Rauf, described as the "key planner" of the terror plot to blow up US-bound airliners, is a close relative of Jaish-e-Mohammed leader Masood Azhar and has been closely associated with the militant outfit for four years, a report from Islamabad said on Thursday.
Rauf is married to a close relative of Azhar, the militant leader set free by India during the 1999 hijacking of the Indian Airlines plane from Kathmandu to Kandahar.
He was arrested in connection with the plot from Bhawalpur, the headquarters of the banned JeM, where Azhar lives, Pakistan daily The Nation reported.
He was arrested only a day before the London terror plot was unearthed.
Rauf, who has dual citizenship of Pakistan and Britain is expected to be extradited to London shortly.
Pakistan's Interior Minister, Aftab Khan Shepao said a team of legal experts from London have arrived to "coordinate and discuss" the legal aspects of extradition.
Rauf's arrest came as Pakistan and UK were on the verge of finalising an extradition treaty. Pakistan officials said the extradition was possible as Rauf was also a British citizen.
Meanwhile, another media report here quoted Hafiz Allah Bukhsh, the father of Azhar as saying that Rauf had joined Al-Qaeda. He "deserted" the movement and joined Al-Qaeda, Bukhsh told reporters at the Jaish headquarters.
"JeM's main cause is Kashmir while Al-Qaeda is interested in securing Afghanistan," he said adding Rauf was in touch with him as he was related by marriage to the family.
The report linking Rauf with JeM came as Pakistan denied a spate of reports about the links between plotters and JUD, a Islamic NGO headed by LeT founder Hafeez Saeed.