Queen's birthday: David Cameron leads tributes to Elizabeth II as she turns 90

Members of the public are expected to sing Happy Birthday when the Queen goes on a walkabout in Windsor

David Cameron has led tributes to the Queen on her 90th birthday, as traditional gun salutes were held across the UK to mark the occasion.

Members of the public were expected to sing Happy Birthday to the Queen as she took part in a walkabout close to her Windsor Castle home.

And in the evening the night sky over her Berkshire home will be lit up by a beacon, started by the Queen, the first in a chain of more than 1,000 that will spread across the country and Commonwealth.

The Prince of Wales is understood to be making a speech in tribute to his mother at the night-time event.

Speaking in a special House of Commons "debate", the Prime Minister told MPs: “Throughout it all, as the sands of culture shift and the tides of politics ebb and flow, Her Majesty has been steadfast - a rock of strength for our nation, for our Commonwealth and on many occasions for the whole world.”

An official photograph, released by Buckingham Palace to mark her 90th birthday, shows Queen Elizabeth II with her five great-grandchildren and her two youngest grandchildren at Windsor Castle (PA)

The birthday has been marked by the release of a photo of the Queen surrounded by her five great-grandchildren - including Prince George and Princess Charlotte - and her two youngest grandchildren.

The image is one of three taken by the American celebrity photographer Annie Leibovitz.

One unusual tribute will see the Houses of Parliament lit up red, white and blue in honour of the Queen's milestone.

The next day, on 22 April, the Queen will have lunch at Windsor Castle with the US President Barak Obama.

In May, a four-night celebration in the grounds of the castle will be held. Tickets sold out within hours for the 25,000-seater event from 12 May to 15 May.

And it will happen all over again around 11 June, the Queen’s official birthday, with a national service of thanksgiving at St Paul’s Cathedral and the Trooping the Colour in Horse Guards Parade.

Additional reporting by agencies