PM's sons send cards to Mandelas

Mr Brown described the former South African president as "the most amazing man" as he explained the arrangement.

Youngest son Fraser, three, celebrates his birthday a day before Mr Mandela while six-year-old John shares a birthday with Graca – "so they send each other cards," Mr Brown told Tesco magazine.

The Prime Minister also said he tries to "keep in touch" with Mr Mandela and spoke to him on his birthday last year.

Mr Brown spoke movingly about how the death of his mother in 2004 affected him.

"You don't quite realise how much you depended on her until she's not there," he said.

"Your mother is so central to everything that you are, as well as what you do and how you behave, that it's very difficult to contemplate life without her."