There is a high possibility that Prince Harry and Meghan Markle’s decision not to invite either UK or international leaders to their wedding is to avoid the presence of US President Donald Trump, a British royal etiquette expert said at a news conference to foreign press in London on Tuesday.

William Hanson, who is widely regarded as the United Kingdom’s freshest and most trusted authority on etiquette and protocol, said Markle had previously voiced her unfavorable opinion of Trump before her engagement, so with the wedding invitations excluding politicians is a diplomatic way to avoid having to invite him.

Instead, the couple have chosen 600 guests with whom they have a direct connection to witness their marriage on May 19 at Windsor Castle’s St George’s Chapel.

Despite Harry and Meghan’s wedding being an important national occasion, the prince is not in the immediate line of succession. As it stands, Harry is fifth in the line to the British throne behind his father Prince Charles, his brother Prince William the Duke of Cambridge, and William’s two children, and Harry will become sixth in line when the Duke and Duchess of Cambridge’s third baby - due very soon - is born.

Hanson said the wedding planning had clearly taken into consideration the arrival of the new baby, which could happen any time.

"Harry is very close to his brother and his sister-in-law, and presumably by extension, Meghan is also part of that circle," Hanson said.

As far as the wedding is concerned, Harry "would not want something to upstage, nor to prohibit, such a major guest from coming", he added.

Hanson, who has been teaching royal etiquette for the last 12 years, half-jokingly said: "This wedding is probably going to be the one wedding that irritates everybody like me, because actually it will break the protocol as Meghan seems to have her own take on what she wants to do."

The wedding cake has been announced well in advance, and the photographer as well as the florist are usually not revealed until much closer to the event, if not on the day or after the day, he added, "so already we are beginning to see, on smaller levels, a different sort of thing".

Hanson said Harry, who achieved the rank of captain in his 10 years of army service, is likely to wear morning dress at the wedding, instead of the military uniform the Duke of Cambridge wore at his big day, because the upcoming wedding is less formal.

According to Hanson, for men, key morning dress rules include a black or dark gray tailcoat with neck tie, and the waistcoat is usually double-breasted. For women, it will be a formal dress with jacket over the top, hats instead of headpieces, plus gloves to be super-correct.

When asked how much Markle’s previous marriage and divorce matter to the royal court, Hanson said it would have been more of an issue if Harry were directly in line to the throne, although the royal family has relaxed their attitude to divorce during the Queen’s reign.

The couple have announced that they will be inviting more than 2,000 members of the public into the grounds of Windsor Castle to watch part of the wedding.