The Queen officially opened the BBC's Broadcasting House in a live broadcast on BBC Radio 4.
During a tour of the building afterwards, she appeared behind TV newsreaders on air.
"It is a great pleasure to visit the BBC today, and to see it in its new home," she said on Radio 4.
"I remember first coming to Broadcasting House with my father, the King, and my mother and sister shortly before the war.
"I came again with the Duke of Edinburgh shortly before the coronation in 1953.
"I hope this new building will serve you well for the future and I am delighted to declare it open today."
Afterwards, she had a tour of the BBC newsroom during a News Channel broadcast.
Staff broke into applause as she appeared in the background of a studio shot, prompting the newreaders to turn round.