The Duke of Sussex's lawyers fired off a "pre-action protocol letter" to the Home Office department of the United Kingdom Government -- think Homeland Security for comparison -- in which they made clear ... if Harry and his family are not offered public security, from police officers and the like, while in the U.K. ... they'll be seeking a judicial review.

The move to essentially threaten the government with legal action reportedly comes after Harry, of late, has offered to pony up the cash for his own police security detail ... which was apparently denied from high-ups in HO, despite Harry insisting it was absolutely necessary.

Harry's reps released a statement explaining why he was doing this, arguing that he has taken on an inherent risk that he acquired from birth ... and which he'll carry for the rest of his life. Now, that he's got a family with Meghan Markle, that risk extends to them as well.

His reps also said this ... even though Harry and Meghan pay for their own private security here in the States -- where they now live since Harry stepped away from his Royal duties -- but unfortunately ... those bodyguards/that intel does not suffice when they visit his native land.

They cite a security breach in July 2021 -- when he was in town for a statue unveiling for his mother -- and once again insist ... he needs to be able to protect himself and his family.

Harry's team goes on to say he wants to bring his children, Archie and Lilibet, to the U.K. to visit ... but because of this ongoing issue -- which has apparently been playing out BTS for at least a couple years now -- he isn't able to, as they say, it's too dangerous.