The captain of an ultra-elite UK nuclear sub armed with doomsday weapons filmed a sex tape with an underling while at sea, officials say.

The officer was in charge of the $5 billion Vanguard-class submarine — which carries Trident Two nuclear arms — when he took the X-rated video of himself and the junior sailor, The Sun reported.

The unnamed captain, who once met Princess Anne, moved on to a desk job — but eventually doomed himself by allegedly sharing the X-rated selfies, which higher-ups caught wind of, the outlet reported.

Crewmates then corroborated the seamy at-sea shenanigans, it said.

The officer, one of the youngest to captain a nuclear-powered sub, was suspended and soon after sacked under the Navy’s “zero tolerance policy” for inappropriate behavior.

The scandal comes as the UK’s Navy suffers a shortage of submarine staff, according to the outlet.

The Royal Navy has four nuclear-armed submarine classes: Vanguard, Vengeance, Victorious and Vigilant — and always has a vessel at sea in case of a nuclear attack.

In a statement, the Royal Navy told The Sun, “All forms of unacceptable behavior are taken extremely seriously and anything which falls short of the highest standards will not be tolerated.

“Anyone who is found culpable will be held accountable for their actions, regardless of their rank or status,” it added.