Princess Diana's former bodyguard says a lone attacker is the biggest security threat to Prince Harry and Meghan Markle's wedding.

His Royal Highness and his Hollywood actress fiancee are set to tie the knot tomorrow at Windsor Castle. They cannot marry at the favoured venue of the royal family, Westminster Abbey, because Markle is a divorcee.

Prince Harry and Meghan Markle (pictured after their engagement) are set to wed tomorrow. Photo / File
Prince Harry and Meghan Markle (pictured after their engagement) are set to wed tomorrow. Photo / File

Ken Wharfe, who served as a royal protection officer for 16 years, told NZME he was pretty confident the wedding would be go off without a hitch, but if there was to be a security incident it would be "from a lone, fixated individual that wants to make a statement".

"We've seen this in the past - you know, the naked runner or somebody who chooses to through something or make a rush towards the carriage. These are the real problems - more of an embarrassing factor than anything else."

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Thames Valley police and the Metropolitan police, who are responsible for security at the wedding, would have been preparing for the big day since Prince Harry and Markle announced their engagement at the end of last year, he told Newstalk ZB's Larry Williams.

Security would be tight and all the guests would be searched and vetted before they could set foot on the castle grounds.

Windsor Castle in Windsor, England, is where the wedding of Prince Harry and Meghan Markle will take place on Saturday. Photo / Associated Press
Windsor Castle in Windsor, England, is where the wedding of Prince Harry and Meghan Markle will take place on Saturday. Photo / Associated Press

The biggest risk would be when the newlyweds left the castle for a procession through Windsor town in an open horse and carriage.

The route they will take is lined with businesses and residential properties and security personnel would have searched each building and spoken to each homeowner before the wedding.

Prince Harry and his fiancee,
Prince Harry and his fiancee, "Suits" actress Meghan Markle, leave Windsor Castle after a day of rehearsals for their impending wedding at the Berkshire Venue. Photo / Australs

"Since William and Kate's wedding a lot's happened internationally. We have this rise of terrorism, so there's a lot more work to be done," said Wharf.

"But I'm pretty confident that with [the police's] expertise I think the country and the world can expect a happy day.

"I'm not suggesting or saying that it will be 100 per cent, no security operation ever is, but with the expertise we can expect there to be, in my view, a very, very closely co-ordinated security operation."

Wharfe also weighed in on what he believed Princess Diana would think of Markle and Prince Harry's union if she were alive.

Prince Charles and Princess Diana with newborn Prince Harry. Photo / Getty
Prince Charles and Princess Diana with newborn Prince Harry. Photo / Getty

"I think that it's very frank that it's irrelevant really that he's marrying somebody of mixed race and American and a divorcee," he said.

"I think [Princess Diana] would be absolutely delighted on the proviso that her son was happy and it clearly looks like to me - and I haven't spoken to him for a few years - that he seems to be very happy and if that works I can't think of a better combination."

Tomorrow night nzherald.co.nz will have the latest coverage of the royal wedding as it happens. The wedding will begin at 11pm tomorrow (midday UK time).

Read more: Prince Harry 'would be a remarkable king' says ex-bodyguard