A 101-year-old man became the oldest Briton ever to be convicted of a criminal offense when he was found guilty on Friday of a string of sex crimes against young girls.

Ralph Clarke, a retired truck driver, was convicted of abusing and raping two young sisters in the 1970s and early 1980s, having earlier admitted indecently assaulting their brother and attempting to rape him when he was an infant schoolboy.

Despite his age, he is likely to be given a prison term when he is sentenced on Monday.

"Clarke subjected innocent children to truly shocking abuse over a prolonged period," said Detective Constable Emma Fennon. "There may be people who question the merits of taking a 101-year-old man to court, but age alone is no defense against prosecution. Those he assaulted had to live with the suffering he inflicted on them for decades."

Police said he was the oldest person to have been found guilty of a crime in British legal history.

Clarke abused the girls, who were between 4 and 13 years old, at his home in Birmingham, central England, in the cab of his work truck and in his garden shed. He also assaulted their brother on numerous occasions.

His victims finally contacted police in August last year, having seen Facebook posts celebrating Clarke's 100th birthday.

Prosecutors decided that despite his age and frail condition he should go on trial because of the severity of the crimes.

Clarke admitted several indecent assaults, gross indecency and attempted buggery against a boy at Birmingham Crown Court but denied assaulting the girls, who are now in their 40s, claiming they were serial liars.

He was unanimously convicted by the jury of more than 20 charges of indecent assault and gross indecency against them.