A legal complaint from an Aussie could block Harry and Meghan's bid to trademark their Sussex royal brand.

The pair want to use the name to launch a foundation and make money from branded goods, The Sun reported.

But they face a battle after a doctor from across the Ditch registered a "notice of threatened opposition" with the UK Government's Intellectual Property Office this week.

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The Sun says the notice was filed by Benjamin Worcester of Victoria, who is believed to have worked as a doctor in London from 2011 to 2014.

Trademark lawyer Lee Curtis told the paper that filing such a notice was free and relatively easy but laying a formal notice of opposition was much more involved.

"The whole opposition could take at least a year to get to a decision and is thus not an action entered into lightly with a possible costs award against the losing party."

The period of opposition for Harry and Meghan's application was due to end on February 20, but that period has been extended until at least March 20.