Three Pakistani men will be prevented from giving evidence to Parliament regarding Britain's alleged role in CIA drone strikes after they were refused entry to the country.

The men, including Noor Khan, who is currently suing Britain over its alleged role in a CIA attack which killed his father, had been invited to address the all-party parliamentary group on drones yesterday.

Also due to address the meeting was Kareem Khan, whose son and brother were killed in a drone strike on New Year's Eve 2009.

Mr Khan is suing former CIA station chief in Pakistan Jonathan Banks and ex-CIA general counsel John Rizzo over the killings.

Noor Behram, a journalist who has been investigating and photographing drone strikes in the tribal areas of Pakistan for almost six years, had also been scheduled to attend. Yet they were prevented from doing so after their visa applications were rejected.

The family of another Pakistani drone victim recently gave evidence to a congressional hearing in the US having been granted visas to do so and campaigners have urged Britain to extend the same courtesy.

Legal justice charity Reprieve strategic director Cori Crider, whose organisation represents Mr Khan, said: "It is an unfortunate coincidence that David Cameron is refusing to grant a visa to the very same man who is suing his government over its role in the drone strike that killed his father.

"Just last week the Rehman family were able to tell their story to the US yet the UK seems unwilling to extend a similar courtesy to these three victims of the drone programme. The British government must reconsider and grant the men visas."

Labour MP Tom Watson, who chairs the all-party parliamentary group on drones, had written letters supporting the three men's visa applications.

Mr Watson said: "It's very disappointing that visas have not been granted in time for the drone victims invited by the parliamentary group on drones to speak today.

"The Rehman family testified to Congress about their grandmother who was killed by a CIA drone.

"The UK must allow Noor Khan and other survivors into the country so that we too can hear these lost voices."