Biden said the Queen asked him about Vladimir Putin and Xi Jinping at their Sunday meeting.
He appeared to breach an unwritten rule that talks with the Queen are private and off the record.
But the Queen may have approved Biden's remarks to support the G7 summit, a royal expert said.
President Joe Biden appeared to break longstanding protocol by revealing what he and Queen Elizabeth II talked about in their private meeting - but there's a chance she approved it for political reasons, one royal expert said.
Biden, who visited the UK last week to attend the G7 summit, met the Queen on Sunday for a private tea session at Windsor Castle. Speaking to reporters after, he was unusually candid about what they talked about, including saying she reminded him of his mother.
He also said she asked him about Russian President Vladimir Putin and Chinese President Xi Jinping, saying: "She wanted to know what the two leaders that I - the one I'm about to meet with, Mr. Putin - and she wanted to know about Xi Jinping," without going into detail.
There is no strict rule about revealing what the Queen says to you, but it goes against established convention to do so, one royal expert told Insider.
"It was certainly a surprise," Richard Fitzwilliams said of Biden's remarks.
"It is the rule that nothing that is said [by] the Queen is repeated," he said. "The whole point of these conversations - like the prime minister's weekly audience with the Queen - is that nothing is revealed."
"So technically, yes, he did indeed breach what is understood to be protocol," Fitzwilliams said, emphasizing that by "protocol" he was referring to informal expectations, rather than any written rule.
However, he floated a theory that Biden's comments may have been "cleared and arranged in advance," he said, to give royal weight to the G7's democratic values.
"My only suggestion is that the reason, possibly, that this was done, was because the G7 sees itself as a bulwark against totalitarian regimes."
Threats to democracy - including relations with China and Russia - was one of the G7's topics of discussion in the leadup to the summit.
The Queen never comments on political matters. But this move could allow Biden "to indicate the Queen had concerns," Fitzwilliam said.
A spokesperson for Buckingham Palace declined to comment.
It's not the first instance of someone breaking the unwritten rule. In 2012, the BBC apologized after its reporter, security correspondent Frank Gardner, said the Queen had told him her views on the radical Muslim cleric Abu Hamza, as The Guardian reported at the time.
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