Duncan Larcombe, who joined independent full-service agency Zest last year, is a former Sun royal correspondent whose book Prince Harry: The Inside Story was published in June.
Speaking to PRWeek, he said that William and Kate, who have become more "serious" and "potentially one-dimensional" in their public persona of late, were likely to be "massively overshadowed" by their more glamorous in-laws.
This presents the royal family with a problem. "How do they keep Will and Kate on the front pages when they’re no longer the biggest show in town? There are definitely some challenges coming up here," he said.
Prince Harry's comms are currently overseen by Jason Knauf, who also looks after the Duke and Duchess of Cambridge. Asked whether it was likely Harry and Meghan would be given a separate comms team once married, Larcombe said: "I would suggest possibly yes - as soon as he is married and they are a double act, he can do all sorts of engagements, foreign vistis, which the Queen quite frankly is too old to do."
This would present a difficult balancing act for Sally Osman, the comms director for The Royal Household, who would have to work to ensure there is "no perception of there being any rivarly" between either the couples or their PR aides.
End of the 'stand-off'
Larcombe said that the release of today's news followed the "same blueprint" as Kate and William's engagement in 2010, where there was a gap between the engagement itself and the announcement, during which the Queen gave her blessing.
"Everyone knew that this was on the cards," Larcombe noted, but said that despite this speculation, and some stories appearing in recent weeks asking whether the engagement had already happened, royal press teams would not have been able to give any off-record briefings to media.
"On engagements, pregnancies, things like that, they don’t offer any guidance, they say 'we can’t comment' so there will have been this weird stand-off and probably they [the PR people] wouldn’t have known about it until after the Queen was told," he said.
Asked whether the wedding would be likely to gift the UK a public holiday, Larcombe said: "I would suggest that is probably not likely - it’s not protocol, but you never know."
Knauf's rising stock
Jason Knauf took up his role in late 2014, succeeding experienced practitioner Patrick Harrison and doing so shortly after the Royal family's press offices were again separated following a trial merger instigated by Osman.
Knauf received criticism by media and commentators earlier on in his stint, and lost his deputy Nick Loughran to Freuds in early 2016.
However, he has since received praise for an uncharacteristically robust response to press coverage of Markle in November 2016, and giving Harry platforms to talk about mental health and grief, in particular around last year's London Marathon.
This has led to improved perceptions of both a royal who was formerly something of a loose cannon who either lacked or did not abide by his comms counsel, and of his PR man.