Britain’s republican movement has revealed it plans to mount a campaign for a referendum on the future of the monarchy when the Queen dies.

Republic has been nervous of appearing callous over the Queen’s mortality, because it sees the popularity of the Queen as crucial to public support for the institution.

But on the eve of the Queen’s 90th birthday, Republic’s chief executive, Graham Smith, suggested that, when it happens, the Queen’s death will mark a turning point in public attitudes.

“The Queen’s birthday reminds us that support for the monarchy is bound up with support for the Queen,” he said.

In a statement to mark the royal birthday, he added: “In a hereditary monarchy the Queen’s age becomes a political issue. Long before the Queen dies the country will need to debate what happens next.”

Facebook Twitter Pinterest Graham Smith. Photograph: Martin Godwin/The Guardian

But Smith went further when he was asked about the prospects of a UK referendum on the monarchy at the meeting of Alliance of European Republic Movements in Madrid over the weekend.

In a recording of the event published on YouTube, Smith said: “When the Queen dies, the moment she is declared dead, Charles is king. So there is no gap. And there is certainly no official plan for a referendum. He is king immediately. The coronation would be about six months later.

“So that will be an opportunity, after the funeral and before the coronation, for us to do some campaigning and say, ‘Hang on a minute, this is the 21st century, if we are going to have a new head of state then perhaps we want to have a vote.’ Then if we have that vote, it can’t just be, ‘Do you want Charles?’ It has to be ‘Which person do you want?’ and it has to be a free and fair election.”

Facebook Twitter Pinterest YouTube footing of Alliance of European Republic Movements meeting

Last July Smith warned delegates at Republic’s annual conference against portraying the Queen’s death as “an opportunity” for anti-monarchists. But in Madrid he admitted: “It is an opportunity to campaign, but it is not going to be an easy campaign. We are going have to work hard to get that referendum.

“That [the Queen’s death] will make a big difference. In the UK a lot of support for the monarchy is to do with the fact that we have had the same Queen for a very long time and most people can’t remember a time when she wasn’t on the throne.”

The meeting, held at the Ateneo de Madrid, heard contributions from leading republicans from all over Europe.

Jon H Leren, the international secretary of Norway’s republican movement, predicted that Spain, where 45% of the population support a republic, would be the next nation to overthrow its unpopular royal family.

He said: “If one of our nations manages to overthrow the monarchical system there we be a domino effect. So Spain, you are closest, you go first and I promise we will come after.”

Smith said there were plenty of reasons for the European republican movement to be optimistic even if opinion polls showed that only about 20% of the UK population backed the idea. “We are going to get to that point where there are no monarchies left in Europe and that’s a dream I want to make happen in my lifetime,” he told delegates.

“Some people think we’ve got the toughest job [in Europe] – the monarchy that is least likely to fall – but I think that we’ve shown over the last few years that there’s is a lot of reason for optimism.”

