Prince George and Princess Charlotte attend private ceremony for Princess Diana's 56th birthday

Prince George and Princess Charlotte have attended a private ceremony for their grandmother Princess Diana, on what would have been her 56th birthday.

The intimate service was held at the Althorp Estate, on the small island where Diana is buried, at her family's property in Northamptonshire.

Prince William is said to have taken his young children to pay tribute to his mother, on the day she would have turned 56 years old.

William's decision to privately take Prince George and Princess Charlotte to their grandmother's grave comes after their uncle Prince Harry criticised the fact that he had to publicly say goodbye to his mother at her funeral.

Diana is buried at her family's estate in Northamptonshire

'My mother had just died, and I had to walk a long way behind her coffin, surrounded by thousands of people watching me while millions more did on television', he told Newsweek.

'I don't think any child should be asked to do that, under any circumstances. I don't think it would happen today.'

Kate was also in attendance at the service, conducted by the Archbishop of Canterbury Justin Welby, as well as Prince Harry and members of Diana's family.

Diana's ex-husband Prince Charles and his wife Camilla did not attend, as they're on official royal duties in Ottawa, Canada, for Canada Day.

The event to mark Diana's birthday comes just under two months before the 20th anniversary of her death on 31st August.

To mark the anniversary, the BBC will broadcast a special documentary about what happened the week after her fatal accident. The programme include appearances from William and Harry, who said they 'owed' it to their mum to open up about her death.

'Part of the reason why Harry and I want to do this is because we feel we owe it to her... I think an element of it is feeling like we let her down when we were younger', William said.

'We couldn't protect her. We feel we at least owe her 20 years on to stand up for her name and remind everybody of the character and person that she was. Do our duties as sons in protecting her.'

William and Harry lost their mum at a very young age

Both William and Harry have been increasingly more open about coping with their mum's passing in recent months, with Harry admitting he only started to deal with his issues after coming back from Afghanistan.

In a recent interview with Forces TV, he said: 'If you lose your mum at the age of 12 you have got to deal with it. The idea that 20 years later I still hadn't really... that 15, 17 years later I still hadn't dealt with it. Afghan was the moment where I was like, "Right, deal with it".'