The Rev Ian Paisley told Tony Blair he was a "fool" when the former British prime minister told him in 2007 he was about to convert to Catholicism.

The encounter is recalled in a television documentary to be broadcast on BBC tonight, during which Dr Paisley speaks about his special relationship with Mr Blair.

Mr Blair, who has family connections in Co Donegal, once told him how his grandmother had instructed: "You must never marry a Roman Catholic and you must never join the Roman Catholic Church, " and how he went to disobey her on both fronts.

Dr Paisley recalls a meeting in Mr Blair’s private room and how, as they were leaving together, the prime minister said: “ ‘Ian, there is something I need to tell you. When the hands of that clock come to 8 o’clock, I will be a Roman Catholic.’

“He said, ‘I did not want you to leave without telling you, I’d rather tell you myself.’ And I said, ‘You are a fool’ and I walked on.”

During the documentary, Paisley: Genesis To Revelation – Face To Face With Eamonn Mallie, Dr Paisley is asked by his interviewer how he would feel if one of his family became romantically involved with a Catholic.

He first jokes that he would have bought a “long cane” and used it, but adds: “I wouldn’t have liked it but we [he and his wife Eileen] would lump it.”