President Biden refused to visit the British Navy’s new flagship aircraft carrier because of the amount of steps — and took “a bit of shine” to Boris Johnson’s wife during his 2021 visit, the former UK prime minister claimed in his upcoming autobiography.

The president and Johnson met ahead of the G7 summit in Cornwall, where the British government had “proudly stationed” its massive $4.3 billion warship, The HMS Prince of Wales, Johnson wrote in his new book, “Unleashed.”

Johnson and his aides had wondered before Biden’s first presidential trip abroad whether the US leader, then 78, was “really as elderly-seeming as his detractors sometimes said,” according to an excerpt from Johnson’s book obtained by The Telegraph.

“His staff told us that he would not in fact be boarding our vast aircraft carrier — which we had proudly stationed in the bay — because it had so many steps; and we wondered what that meant about his physical fitness,” Johnson said.

Biden did, however, seem spry enough to joke with Johnson’s new wife, then-33-year-old Carrie, whom he appeared to “take a bit of shine” to, the former prime minister wrote.

Biden, who attended the summit with his wife Jill, said both he and Johnson had “married above ourselves” — and joked he and Carrie sneak off for a walk alone.

“‘Why don’t you and I go down to the beach … and leave this guy here?'” he quipped to Carrie as “Jill Biden rolled her eyes,” Johnson wrote in his memoir.

Biden’s 2021 G7 trip was marked by several embarrassing blunders, including a moment during a roundtable discussion where he corrected Johnson for not introducing “the president of South Africa” — even though Johnson already did so by name.

The president bowed out of his reelection race in favor of Vice President Kamala Harris amid concerns about his mental and physical capabilities following his disastrous debate with former President Donald Trump in June.

Johnson’s autobiography will be released on Oct. 10.