Britain holds a National Service of Thanksgiving in honour of Queen Elizabeth on Friday, kicking off a weekend of events to mark the official 90th birthday of the country's longest-reigning monarch.

The church service at London's St Paul's Cathedral, attended by a host of dignitaries including other members of the royal family and Prime Minister David Cameron, will be a double celebration for the queen with Prince Philip, her husband of 68-years, marking his 95th birthday.

"Most especially today do we give thanks for the length of years that has been granted to our most gracious Sovereign Lady, Queen Elizabeth: for her faithful devotion, dutiful commitment, loving leadership, gentle constancy, royal dignity and kindly humanity," David Ison, the Dean of St Paul's, will say.

The queen, who has been on the throne for 64 years and is by far the oldest monarch in British history, turned 90 in April but in keeping with tradition to ensure celebrations take place in the summer, she also has an official birthday usually in June.

To mark the occasion, the sixth and final portrait in the series of official birthday pictures taken by US photographer Annie Leibovitz was released, showing the monarch with her husband at her Windsor Castle home, west of London.

Then on Saturday Elizabeth will attend the traditional Trooping the Colour parade of soldiers in ceremonial uniforms in central London followed by a fly-past of Royal Air Force aircraft over Buckingham Palace.

To conclude the celebrations, the queen hosts "The Patrons Lunch" for 10,000 guests at the largest street party to be held on the Mall, the grand avenue that leads to Buckingham Palace, an event organised by her grandson, Peter Phillips.

The guests will hail from some of the more than 600 charities and other organisations of which she is patron with the festivities beamed to large screens in the capital's parks.

The government also said millions of Britons were expected to join in the occasion at local street parties being held across the country.