Heads of state, high commissioners and foreign ministers have begun arriving at Buckingham Palace ahead of the Queen's opening of the Commonwealth Heads of Government Meeting (CHOGM).
A spectacle of pomp and pageantry is being staged to welcome the dignitaries, with a guard of honour and flag bearers displaying the flags of the Commonwealth's 53 member states waiting to greet them.
Amid tight security, which has seen the streets around the Queen's official London home cordoned off, motorcades began to drive into the palace forecourt.
More than 100 officers and soldiers from the Coldstream Guards are in the honour guard, wearing their famous scarlet tunics and bearskins, alongside the Band and Corps of Drums of the Coldstream Guards.
During the opening ceremony Theresa May is expected to thank the Queen for her life-long devotion to the Commonwealth affectionately known as the family of nations.
Later in the palace's ballroom presidents and prime ministers from across the globe will gather for the launch of the two-day summit, normally held every two years, to discuss the Commonwealth's most pressing issues.
The Prime Minister will address the gathered delegates, as will the Queen and Prince of Wales, and she is expected to say about the Queen: "This week you have opened your homes to us - here in London and in Windsor. Over many years you have been the Commonwealth's most steadfast and fervent champion.
"You have been true to the deepest values of the Commonwealth - that the voice of the smallest member country is worth precisely as much as that of the largest; that the wealthiest and the most vulnerable stand shoulder to shoulder.
"You have seen us through some of our most serious challenges. And we commit to sustaining this Commonwealth, which you have so carefully nurtured."
At the end of their deliberations the leaders usually agree a communique on a major issue and the prime ministers and presidents may commit themselves to a blue charter on ocean conservation.
Other topics expected to be discussed range from cyber security to trade between Commonwealth nations.
Downing Street said on Monday that a decision on whether Charles should succeed his mother as Commonwealth head was expected from the heads of state on Friday, and reports have stated the mood is Charles will get their backing.
Members of the royal family who will attend the opening ceremony include the Duchess of Cornwall, Duke of Cambridge, Prince Harry, Duke of York, Princess Royal, Countess of Wessex and minor members of the royal family
Baroness Scotland, the Commonwealth Secretary-General, and Malta's Prime Minister Joseph Muscat, the out-going Commonwealth chair-in-office, will also address the guests who will include High Commissioners, Foreign Ministers, representatives from Commonwealth organisations, faith leaders and CHOGM forum delegates.