William: I'll shoulder royal duties when the time's right

The Duke of Cambridge has defended himself over criticism that he does not complete enough royal duties.

Speaking to the BBC to mark the Queen's 90th birthday, he said he was focusing on being a father and an air ambulance pilot but would be the "first person to accept" duties when it came for his grandmother to hand over responsibilities.

William also paid tribute to his grandmother's sense of duty and her role as head of the Royal Family.

The second-in-line to the throne said he was making the most of time to do another "worthwhile job" while he could and his family was supportive.

He said: "I take duty very seriously. I take my responsibilities very seriously. But it's about finding your own way at the right time and if you're not careful duty can weigh you down at a very early age.

"I think you have got to develop into the duty role."

In a separate interview he credited the Queen for helping him through the loss of his mother, Diana, Princess of Wales, saying she had been a powerful female influence in his life.

He also said his grandmother had been "incredibly supportive" of him and someone for him to look up to.

Speaking to Sky News for a documentary called The Queen At 90, to celebrate the monarch's birthday, William recalled how a childhood telling off from The Queen stayed with him throughout his life.

He said: "She's been a very strong female influence and having lost my mother at a young age, it's been particularly important to me that I've had somebody like the Queen to look up to and who's been there and who has understood some of the more, um, complex issues when you lose a loved one...

"So she's been incredibly supportive and I've really appreciated her guidance."

Referring to a childhood incident which landed him in hot water with the Queen, William described getting into trouble with his cousin Peter Phillips after riding a quad bike at Balmoral.

He said: "We were chasing Zara around who was on a go-cart, and Peter and I managed to herd Zara into a lamppost. And the lamppost came down and nearly squashed her, and I remember my grandmother being the first person out at Balmoral running across the lawn in her kilt. (She) came charging over and gave us the most almighty bollocking, and that sort of stuck in my mind from that moment on."

The documentary will be shown on Sky News at 9pm on Wednesday.