Elizabeth, 95, the world's oldest and longest-reigning monarch, will mark her seventh decade as sovereign next month and on Monday Buckingham Palace detailed plans for four days of celebrations for her Platinum Jubilee in June.
But anti-monarchy group Republic used the occasion to say it would begin a "Not Another 70" campaign to call for an end to the historic institution.
"While a vocal minority will want to celebrate the queen's seventy year reign, we must all start looking to the future. The prospect of King Charles is not a happy one, and there is a good, democratic alternative on offer," Republic's Graham Smith said.
"It's time to have a serious debate about our constitution, accept that Charles is not the best the country has to offer, and that as a nation we are quite capable of choosing our head of state."
Polls indicate the vast majority of people in Britain support the monarchy and the queen herself is hugely popular. But there is not as much support for her eldest son and heir Charles, and surveys suggest there is growing republican sentiment among younger Britons.
(Reporting by Michael Holden; editing by Guy Faulconbridge)