Britain's Prince William has praised the "strong influence" of his grandmother, Queen Elizabeth II, following the death of his mother, Princess Diana, in a car crash in 1997.

"She's been a very strong female influence and, having lost my mother at a very young age, it has been particularly important for me that I had somebody like the queen to look up to," the 33-year-old prince, also known as the Duke of Cambridge, told Sky News.

He said the queen, who turns 90 on Thursday, had "been there and has understood some of the more complex issues when you lose a loved one."

"So she's been incredibly supportive and I've really appreciated her guidance," said William, who is second in line to the throne behind his father, Prince Charles, 67.

William also told the BBC in a separate interview that the queen had been a "guiding example" to him.

Diana died in a car crash in Paris in August 1997, when William was 15 years old. She had separated from Prince Charles in 1992 and the couple's marriage was dissolved in August 1996.

William features in an official portrait of the queen released by the royal family to mark her 90th birthday. He is shown sitting next to the queen with his 3-year-old son, Prince George, and Prince Charles.

The queen left Windsor Castle, her residence outside London, to open a bandstand in Windsor's nearby Alexandra Gardens, where schoolchildren sang "Happy Birthday, dear Your Majesty."

With her husband, Prince Philip, she also visited a local mail office to inspect a set of four commemorative postage stamps featuring images of the queen, Prince Charles, Prince William and Prince George taken from the official portrait.

Prime Minister David Cameron is scheduled to pay tribute to the queen in parliament on Thursday.

The queen plans to meet members of the public during what her office calls a "Windsor walkabout" on Thursday afternoon.

She will reportedly eat lunch at Windsor Guildhall with a group of fellow 90-year-olds, and receive a birthday cake made by Nadiya Hussein, a British Muslim woman who won last year's final of The Great British Bake-off, a BBC reality show.

In the evening, the queen will light the first of hundreds of beacons to mark her birthday across Britain.

Following Thursday's celebration of her actual birthday, more events are planned in June to mark the queen's official birthday, including a service at London's St Paul's Cathedral and a street party for several thousand people.

Born in 1926, the queen has ruled since 1952 when she was 25 years old.

In September, she surpassed the six-decade reign of Queen Victoria, her great-great grandmother, from 1837 to 1901.

Some royal watchers said Queen Elizabeth's enduring popularity comes partly from her success in avoiding public controversy.

Britain's system of constitutional monarchy keeps the queen as a largely ceremonial head of state, allowing the government to pass legislation and run the country.

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