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Plans by the Duke and Duchess of Sussex to trademark their Sussex Royal brand could face an official challenge.

A "notice of threatened opposition" was filed with the Intellectual Property Office, giving the complainant a month to lodge a formal objection.

That is now being rescinded, after an Australian doctor named in the filing said his details had been used without permission.

But since then, three more notices have been filed.

It is not yet clear on what basis the complainants might want to oppose the Sussex Royal trademark.

Prince Harry and Meghan's foundation applied to trademark the term - used on their website and social media - in June last year. The application covered products such as clothing and stationery, as well as campaigning and charitable fundraising.

Their plans attracted further attention after the couple announced their intention to "step back" from royal duties and become "financially independent".

Records at the Intellectual Property Office (IPO), which governs trademarks in the UK, show that a notice of threatened opposition was filed on 21 January.

The World Trademark Review website said it was filed .

But the IPO said it was rescinding this notice after it had been "advised by an individual that their personal details have been used without their permission to submit a 'Notice of threatened opposition' to the Sussex Royal trade mark".

Since the objection was first publicised, three similar notices also appear to have been filed on 24 January.

The notices mean the "opposition period", during which detailed objections can be made, is extended to 20 March.

Ben Evans, senior associate and chartered trademark attorney at Blake Morgan, said the IPO had considered the Sussexes' application for an unusually long time before publishing the trademark, perhaps because of rules over the term "royal".

He said someone might formally oppose a trademark registration because they have a competing brand with which it might be confused, or because they object to the description - since the couple are no longer senior royals.

But unlike a full objection, filing a notice of threatened objection requires no fee and no evidence, he said.

"You could just do it to be difficult," said Mr Evans. "It looks like the IPO might be quite busy on this one."

It is not the first time the Sussexes have found themselves in a battle over their preferred brand.

The Sussex Royal name on Instagram was originally taken by driving instructor Kevin Keiley, who lives in West Sussex and supports Reading FC - nicknamed the Royals.

Instagram handed the name to the duke and duchess, leaving Mr Keiley as @_sussexroyal_ instead.