Graham Smith told Express.co.uk that the departure of Meghan Markle and Prince Harry has left the monarchy with very few recognisable members of the Royal Family to push to the forefront. Republic is an organisation that campaigns for the monarchy to be abolished and for the Queen to be replaced by an elected head of state.
"People are not interested in Beatrice and Eugenie.
"They are running out of people and I think they are facing a serious problem in the future because it is just going to be Charles and William."
Prince Harry and Meghan Markle stepped down as senior members of the Royal Family in March 2020.
Sophie, the Countess of Wessex, has taken on more public engagements since the Sussexes departure.
Prince Charles has widely publicised plans to slim down the monarchy when he takes the throne from Queen Elizabeth II.
In June, royal commentator Natalie Oliveri told 9Honey that Prince Charles' plan not to grant Archie a Royal Family title when he becomes king is part of his larger aim to slim down the monarchy.
She added that the Prince of Wales does not want the UK taxpayer paying millions and millions of pounds to fund a bloated monarchy.
Ms Oliveri said: "In 2012 the Queen actually changed it which entitled all the Cambridge children to be known as HRH rather than just George so they are also included in this at the moment.
"What it would do is that it would remove Archie's entitlement to be a prince.
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"Under the current rules, Archie is entitled to become a prince when Charles becomes king.
"But Prince Charles is reportedly planning to change that.
"It is all part of his plan to slim down the monarchy."
She added: "Charles does not want that taxpayer footing this huge bill because it is millions and millions of pounds every year."