In her annual address, Queen Elizabeth - who is head of the Armed Forces - is expected to pay tribute to British troops serving in Afganistan.
Her Majesty will offer a message of condolence to those families who have lost their loved ones in the conflict.
She will also outline the need for the Commonwealth to put on a united front against the effects of climate change, a week after global leaders met in Copenhagen to talk about the issue.
The 83-year-old monarch, whose message will be broadcast at 15.00 on BBC1, is expected to outline "issues that concern us all".
Delivering her address from Buckingham Palace's White Drawing Room, she also hopes to give a voice to the young people of the Commonwealth, which celebrates its 60th anniversary this year.