The Queen is to open the Commonwealth Heads of Government Meeting during a ceremony at Buckingham Palace later.
Prime Minister Theresa May will thank the Queen for her “service, dedication and constancy” to the 53 nations.
Issues under discussion at the two-day summit include ocean conservation, cyber security, trade and the Queen’s successor as head of the Commonwealth.
The PM said the issue of plastic waste would be “central” to the UK’s agenda at the bi-annual meeting.
Mrs May said she would be “rallying” Commonwealth countries to follow the UK’s fight against marine plastics – which has included a ban on microbeads and proposals to introduce a deposit return scheme for drinks containers in England.
“The Commonwealth is a unique organisation, with a huge diversity of wildlife, environments and coastlines,” she said.
The UK is due to consult on proposals to ban plastic straws and cotton buds – 8.5bn of which are thrown away in the UK every year – as part of plans to tackle the issue.
It comes as 60 UK music festivals, including Bestival in Dorset and Boomtown in Hampshire, also pledged to ban plastic straws at their events this summer.
As head of the Commonwealth, the Queen presides over the meeting but does not take part in the leaders’ discussions.
A decision on whether Prince Charles will succeed her as head of the Commonwealth is expected from the heads of state on Friday.
Camilla, Duchess of Cornwall, Prince Harry and Prince William are among the royals to attend the opening ceremony.
During the opening ceremony, Mrs May is expected to say the Queen is a “steadfast champion” of the Commonwealth.
“We commit to sustaining this Commonwealth, which you have so carefully nurtured,” she is to say.