Patrick Casement, the closest living relative of Sir Roger Casement, who was executed for treason for his role in the Easter Rising, said yesterday that his family's shame had lingered for two generations.

Mr Casement, who lives in the family seat in Northern Ireland, said his grandfather and father never spoke of their second cousin and he only discovered the connection when he went to school.

"Sir Roger was the black sheep of his family," said Mr Casement, 51. "It was an appalling tragedy and disaster for the family ... My grandfather and his four brothers were all fighting for the British in the war and to be related to Sir Roger was a huge disgrace.

"To have a member of their family being arrested as a traitor, taken off to the Tower of London and tried completely blighted the family's perception of Sir Roger for two [or] three generations."

Sir Roger, knighted for his campaign against the exploitation of plantation workers, was hanged in Pentonville Prison on August 3, 1916, after being found guilty of enlisting Germans to fight against British rule in Ireland. A campaign for clemency failed after his private diaries came to light, exposing his rampant homosexuality. But many have since claimed that the "Black Diaries" were forged by British intelligence in order to destroy Sir Roger's reputation.

Now a team of forensic scientists, commissioned by Goldsmiths College, London, has analysed the handwriting, ink and paper for the first time in 86 years and concluded that they are genuine. Their report will be examined tonight in a documentary, Secrets of the Black Diaries, to be shown on the digital channel BBC4.

Mr Casement said: "The result doesn't bother me. I always felt the diaries were, in a sense, a distraction.

"They have overshadowed [Sir Roger's] real achievements, his humanitarian work. That's what made him a remarkable and extraordinary person. His sexuality was completely irrelevant."