Prince William and Princess Kate are reportedly "terrified" that Prince Harry and Meghan Markle could be seen as part of the royal family, despite the couple stepping back from their roles four years ago. The concerns have been heightened following the Sussexes' successful trip to Nigeria earlier this month, which some have likened to an official royal tour.

Royal expert and author Tom Quinn told The Mirror, "The senior royals, especially William and Kate, are terrified of one thing above all others: that Harry and Meghan will continue to organise trips across the world until they are seen as the public face of the royal family." Quinn added that the fear is exacerbated by the fact that "William knows that with his wife stricken with cancer he cannot compete and he can't stop the Meghan and Harry rollercoaster."

The Sussexes' trip to Nigeria, during which they were greeted with dances, receptions, and visits to schools and charities, has reportedly angered the royal family, particularly Prince William. Quinn explained, "William is absolutely furious and determined to find a way to stop this happening in future. Charles is said to be angrier than anyone has ever seen him."

The royal expert argued that the Nigeria visit might have been designed to give the impression that Harry and Meghan were still working royals, stating, "Everything you might expect from an official Royal visit was there - the receptions, the visits to schools and charities, to wounded soldiers and the disabled. Meghan and Harry's speeches and their whole attitude has been designed to give the impression that they are still fully paid-up royals and William and his father King Charles don't like it one bit."

The concerns over the Sussexes' growing influence come at a time when the royal family is set to postpone engagements "which may appear to divert attention or distract from the election campaign" over the next six weeks ahead of the general election.

In addition to the Nigeria trip, Prince William and Kate Middleton are also reportedly "nervous" about Prince Harry's potential visit to the U.K. in May to celebrate the 10th anniversary of the Invictus Games.

While it was initially reported that the couple did not want to see Harry, royal expert Tim Quinn suggests that they are more afraid of the drama and speculation that would come along with avoiding Harry and Markle altogether.

"William and Kate are really nervous that whether Harry comes alone or with Meghan there are going to be difficulties and a great deal of planning is going on to try to reduce all the inevitable awkwardness and embarrassment," Quinn told The Mirror.

He added that the couple would rather put their differences aside and see Harry (and possibly Markle) briefly in order to keep rumors and theories at bay, describing it as "a question of the lesser of two evils."

The ongoing tensions between the royal family and the Sussexes have been further highlighted by reports that King Charles is "angrier than anyone has ever seen him" over the couple's Nigeria trip. Royal author Tom Quinn told The Mirror that both King Charles and Prince William are "furious" about the trip, which they perceive as the Sussexes acting as if they don't need permission to be working royals.