Prince Harry and the Duke and Duchess of Cambridge will join thousands of sporting fans for the opening ceremony of the Rugby World Cup.
The royal trio will be at Twickenham when England, who are hosting the tournament, play in the opening game against Fiji.
Prime Minister David Cameron, who described the World Cup as a "compelling spectacle", will also attend.
He said: "The tournament promises to be the best ever and it will leave a lasting legacy for the sport in our country and around the world.
"The sporting drama of the coming weeks will inspire tens of thousands more people to pick up a rugby ball and play the game, and the RFU is investing millions more in clubs up and down the country, training more coaches, and reaching out to make sure rugby is played in more schools.
"The whole country will join with the cities hosting games in England and Wales to offer the warmest of welcomes to visiting teams and fans - and I wish all four Home Nations the very best of luck."
On Thursday Mr Cameron told an audience of business leaders at an event to boost trade links around the World Cup: "I hope you are going to see an extraordinary festival of sport in the coming weeks and we are so proud to be the host. Once again we want to demonstrate that Britain is the best host for sporting events in the world."
The Prime Minister insisted members of his Government are "not going to sit back and enjoy too many games of rugby" but added that he was going to the England v Fiji game as a "special treat".
Details about the opening ceremony are not being released ahead of the event but it is likely to reflect England's long rugby heritage and Harry will give a speech.
The prince is honorary president of England Rugby 2015, the organising committee staging the tournament, and at a welcoming party for the 20 competing nations on Thursday night he spoke about his love for the sport.
He said: "As a passionate rugby fan, I will share in the rollercoaster of emotions with millions of other fans around the world between now and 31st October.
"It is this rollercoaster that makes sport - and especially rugby - so compelling.
"I have no doubt that all the highs and lows, the drama and tension, will create unforgettable memories."
He added: "I believe that this World Cup - more than any before - has the potential to grow the game, both at home and abroad, by inspiring children of all backgrounds, and encouraging men and women of all ages and abilities, to embrace rugby."
The celebrations were held at the Foreign and Commonwealth Office, which the Prime Minister described as a "splendid building".
He joked: "You can fit number 10 Downing Street in here about five times over", adding: "I'm going to arrange a job swap one of these days."
The Prime Minister also revealed that former England captain Martin Johnson had made a lasting impression when he earlier visited Downing Street, damaging a 200-year-old chair.
Mr Cameron said he had offered Johnson an "important seat" but when he sat back "the thing snapped in two".
When Johnson asked if it was an "old chair", the Prime Minister replied: "Well it had been sat here for 200 years but I'm sure we can find another one".
Labour leader Jeremy Corbyn will not be attending the opening ceremony. A Labour spokeswoman said: "No, he is not going. He has pre-existing commitments."
Members of the Royal Family will be supporting the six-week World Cup with Harry and the Princess Royal joining the Queen and Duke of Edinburgh at a Buckingham Palace reception in October for the competing nations and some of the tournament organisers.
William will watch a number of matches with Kate in his role as vice royal patron of the Welsh Rugby Union, and on Monday the Duke will attend the Welsh team's welcome ceremony at the Guildhall in London.
World Cup fans will be joined by the Princess Royal, patron of the Scottish Rugby Union, and her husband Vice Admiral Sir Timothy Laurence at a number of matches.
Competing Commonwealth countries will receive support from other members of the Royal family, including the Duke of Edinburgh, Princess Eugenie, and the Duke and Duchess of Gloucester.