Queen Elizabeth, who celebrates 60 years on the throne this year, will open both the London Olympics and Paralympics, Buckingham Palace and organisers said on Wednesday.
The third Olympic Games to be held in the British capital start on July 27 while the Paralympics opening ceremony at the same stadium in east London will be in exactly six months' time on August 29.
The monarch also opened the Montreal Olympics in 1976 and her husband the Duke of Edinburgh opened the 1956 Melbourne Games on her behalf.
This, however, will be the first time she has opened a Paralympics.
The Paralympic Games will undoubtedly be a time of extraordinary pride for our nation, and it is fitting that Her Majesty will declare us underway, London 2012 chairman Sebastian Coe said in a statement.
London previously hosted the Olympics in 1908 and 1948 and will be the first city to stage the Games three times.
The Queen's father, George VI, opened the 1948 Games at the old Wembley Stadium and her great-grandfather Edward VII performed the honours at the now-demolished White City Stadium in 1908.
(Reporting by Alan Baldwin, editing by Ed Osmond)