Some 160 guests - including injured military personnel, veterans and relatives of those killed - attended the event in Downing Street.
Dame Vera Lynn, who was known as the Second World War "Forces Sweetheart", was also there.
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Hide AdThe Prime Minister said: "This anniversary year is a time for us to remember just how much the Royal British Legion do to care for our armed forces and their families, from those who served in the first two world wars to those who are serving now in Afghanistan.
"While their needs have changed over that time, the need for the British Legion has not - the care and support they provide is as vital now as it was in 1921 and I'm delighted to support their work."