Palace officials will want the trip to be positive and forward-looking, but they'll be all too keenly aware of how last year's Caribbean tour by Prince William and Catherine was overtaken by arguments over the long shadow of slavery.

Protesters held up banners saying "Apologise", demanding formal royal recognition of the historic wrongs of the slave trade.

But royal historian Prof Heather Jones says that even if the King wanted to deliver his own symbolic apology, beyond the "personal sorrow" that he has already expressed, he would need the approval of the government.

"As a constitutional monarch, he is constrained in what he can say publicly," says Prof Jones, of University College Londo...