MPs must challenge and openly oppose Prince Charles if he insists on commenting on political issues. That's the response of campaign group Republic to news that Charles has been speaking out on the question of radicalisation.

Graham Smith, Republic's CEO, has called on all MPs to start treating Prince Charles like any other activist or commentator.

Today he said:

"The real danger is if MPs and ministers feel unable to question or oppose what Charles says. If Charles is free to intervene on the most controversial and sensitive issues of the day without being challenged then we face a very serious problem."

"The reluctance to challenge the royals is based on the fantasy of royal impartiality. Charles is not an impartial or neutral figurehead, he is clearly establishing himself as a political activist and commentator."

"Ideas need to be debated, questioned and opposed - that's how modern societies develop their understanding and responses to difficult issues."

"We cannot have a situation where the heir to the throne is able to speak on such controversial issues as radicalisation and terrorism without his ideas being subjected to the same level of scrutiny and challenge as others would face."

"MPs in particular need to face up to the new reality of an activist prince and must be free to openly challenge what Charles says."

"Many people may sympathise with Charles's views, but that doesn't give him the right to go unquestioned, unchallenged or unopposed. Those who disagree with him must speak out."