Britain celebrated the 90th birthday of Queen Elizabeth II on Thursday with tributes to a popular monarch who has steered it through the decline of empire and a wave of scandals to the Internet age.

Prime Minister David Cameron paid tribute to the queen as "a rock of strength for our nation" and her son and heir Prince Charles marked the occasion by reading Shakespeare verses in a special broadcast on the BBC radio.

The sovereign, who last year overtook her great-great grandmother queen Victoria to become Britain's longest-reigning monarch, will meet well-wishers near Windsor Castle later on Thursday to mark the day.

In the evening, she will light a beacon, the first in a chain of 1,000 which will be illuminated around the country and the world, and host a family dinner.

While still an active monarch, the queen, who has reigned since 1953, has scaled back her duties in recent years as Prince Charles and grandson Prince William plus wife Kate take a more prominent role.

The royals remain popular but some analysts question what will happen when the queen -- who is in good health, enjoying regular rides on her favourite pony and walking her corgis -- reaches the end of her life.

Events to mark the 90th birthday of Queen Elizabeth II today

The Times newspaper in an editorial praised the queen as "a symbol of continuity and the best embodiment we have of a complex national identity".

But it warned the monarchy "will not long retain its popularity" if Charles, an opinionated environmentalist, is seen as interfering in politics in breach of constitutional conventions.

Part of the queen's appeal in Britain is that she is seen as above the political fray in Westminster, while Charles's views often prove controversial.

New family photos

British MPs will devote most of the day to a debate in her honour led by Cameron in the House of Commons and parliament will be lit up in red, white and blue, the colours of Britain's flag.

Two military gun salutes will take place at London's Hyde Park and the Tower of London to mark the occasion.

Buckingham Palace released three new official pictures of the queen taken by US celebrity photographer Annie Leibovitz.

They were shot at Windsor Castle, where she will host US President Barack Obama and First Lady Michelle Obama on Friday.

The queen is pictured with four of her beloved dogs outside the mediaeval castle in one photograph, and with her only daughter Princess Anne in another.

Supporters wish Queen Elizabeth II a happy 90th birthday as they wait in hope of seeing her following her engagements in Windsor on April 20, 2016. Photo: AFP

In the third she is surrounded by some of the youngest members of her family, including her two-year-old great-granddaughter Mia Tindall, who is shown clutching one of the queen's trademark handbags.

Prince Charles read an extract from William Shakespeare's "Henry VIII" about the future Elizabeth I in tribute to his mother on BBC radio.

"She shall be, to the happiness of England/ An aged princess," read the text, which also described her as "a pattern to all princes living with her/And all that shall succeed".

Most of Britain's newspapers carried pictures of and tributes to the queen Thursday.

"Happy And Glorious" was the headline on The Times, though its cartoon hinted at wider questions over the prospect of Prince Charles eventually taking over.

A speech bubble coming from the balcony of a smoggy Buckingham Palace said: "You won't get me out... I'm too worried about the heir quality!"

Not a foot wrong?

The royal family endured what the queen called an "annus horribilis" or horrible year in 1992 when Prince Charles separated from his first wife Diana, while two of the queen's other children also went through breakups and Windsor Castle was hit by fire.

Its popularity plunged after Diana died in a Paris car crash in 1997 and the queen was initially accused of reacting coldly.

Advisors subsequently steered the royals towards a more modern image.

(L-R) Prince Charles, Queen Elizabeth II, Prince George and Prince William during a photo shoot for the Royal Mail in 2015 in the White Drawing Room at Buckingham. Photo: AFP

William and Kate plus their two children George and Charlotte are now, along with the queen, the best-regarded royals.

Royal fan John Loughrey, who has been sleeping on a bench outside Windsor Castle since Monday to be there for the big day, said the queen "modernises" the monarchy every 10 years.

"She hasn't put a foot wrong," Loughrey, who was dressed in red, white and blue, said before launching into a rendition of the national anthem.