Charles will have one key thing to get right as 'transitional' king after Queen dies

Prince Charles has one particular challenge to overcome when the Queen dies and he becomes king, claims a royal expert.

The Prince of Wales is already 73 but when his long-serving mother passes away he must "modernise" the Firm, according to ex-Vanity Fair editor-in-chief Tina Brown.

The journalist and author said the first-in-line will be a "transitional figure" before being succeeded by his son Prince William.

Ms Brown - who last month published The Palace Papers: Inside the House of Windsor, the Truth and the Turmoil - told LBC's Tonight with Andrew Marr, the public will "rally" around him.

She said people will afford him something of a grace period because "it's such a big moment", but quickly "it's a question of how he manages that honeymoon".

Charles must move to modernise the Firm to help transition to William, says Tina Brown (Getty Images)

Speaking to the radio station on Thursday evening, Ms Brown said: "He's got a huge challenge because he has to modernise, he has to now prepare the path for William.

"That is really his prime function, in his what will be a much shorter reign, obviously, which is to get things right for when William comes in."

Prince Charles' eldest son the Duke of Cambridge turns 40 in June.

Prince Charles is already 73, so his reign is likely to be short (Getty Images)

Ms Brown continued: "Now, I don't know whether he will see that as his role but that is his role.

"He's got to do the modernising...He's a transitional figure.

"He’s either going to get it right or he's going to get it wildly wrong.

"We have yet to see, but, of course, that's what makes them so interesting to write about."

Going on to talk about what impact the Queen's death could have on British self-identity, Ms Brown said it's going to be a "pivotal point for this country".

Prince Charles who is the longest serving heir apparent in British history (Hampshire Live / Darren Pepe)

She said Britain has "already had lots of self-identity questions" following three years of Brexit, with "wounds of that...still kind of festering".

She added: "It's a moment of national trauma and then the Queen dying, she's been on the money for 70 years, you know?

"I mean, three generations have known nothing but the Queen so, you know, I do anticipate a very kind of, a moment when everything is going to be very tremulous here."