Terror suspects Ibrahim Adam and Mohammed Azmir are thought to have died in a CIA missile strike on Waziristan, a remote and lawless mountainous region along the border with Afghanistan.
A close friend of Azmir's family, who did not want to be named, said: 'They have taken it very badly - this is the second son who has been killed in a drone strike.'
It is not known precisely when the men died, but it is thought to have been at least three months ago, according to a source with personal connections in Pakistan.
Adam, 24, from Barkingside, east London, had been on the run from the UK authorities since absconding from a control order in May 2007.
His brother, Anthony Garcia, was jailed for life in April 2007 for his part in a major fertiliser bomb plot to attack targets in London and across the UK.
Adam was made subject to a control order after being stopped while en route to Syria after the British authorities claimed he was planning to travel to Iraq or Afghanistan for jihadist training or to fight Western forces.
Father-of-three Azmir, 37, who was born in Sheffield and lived in Ilford, Essex, was made subject to a Treasury order freezing his assets in February 2010 in response to concerns he was involved in funding terrorism.
He is not currently on the Treasury's list of people and organisations subject to financial sanctions.
It is believed that Azmir's brother, Abdul Jabbar, 32, was killed in an earlier CIA unmanned drone attack in Pakistan in September last year.
The Foreign and Commonwealth Office could not confirm Adam and Azmir's deaths but said it was investigating the reports.
A spokesman said: 'We are aware of reports and are looking into them further.' The Home Office declined to comment.
The CIA's drone programme in Pakistan, which is not publicly acknowledged by the U.S., is hugely controversial.
The Pakistani authorities have criticised the attacks as violations of their country's sovereignty and international human rights campaigners have condemned reports of innocent civilian casualties.
Cori Crider, legal director of campaign group Reprieve, said: 'As with all of the CIA's drone campaign in Pakistan, these deaths are shrouded in mystery.
'What we need to know is what role the British Government may have played in the killing of two of its own nationals.
'Did UK officials provide the US with information which helped them to target their attack on these two people?
'If so, is Britain now complicit in the CIA's illegal campaign of drone attacks, which has resulted in hundreds of civilian deaths?
'We also need to remember that, while it is unusual to hear of British casualties, these strikes are an all too frequent reality for the people of north-west Pakistan.
'We need to do everything that we can to shine a light on this murky and counter-productive CIA campaign, which is killing civilians and fuelling instability in the region.'
American officials have privately said that the strikes have killed many Taliban and al Qaeda commanders.