Joe Biden's decision to drop out from the 2024 presidential election Sunday didn't just shock the world ... it also shocked members of his own staff, according to a new report.

Prominent political publication Politico cites an anonymous Democrat who they spoke to after the sitting president announced he would drop out of the 2024 election.

The official told Politico, "We’re all finding out by tweet. None of us understand what’s happening."

It's a shocking revelation ... like we told you, NBC News reported Biden was discussing dropping out with family -- and, one of their main concerns was letting JB drop out in his own time without a concrete plan in place.

However, if White House aides were shocked by the news, then it seems the transition plan wasn't fully in place at all ... making it seem Biden made something of a snap choice.

Other updates from Politico ... they say Senior White House Advisor Anita Dunn held an all-staff comms call after the announcement -- and, an aide told the publication she "reassured everyone who had been out front saying this wasn’t happening that they were correct because the decision came down late." So sounds like even she may have been a bit blindsided.

Now, the Biden-Harris campaign's shifting gears ... backing VP Kamala Harris who President Biden endorsed this morning -- and, who released a statement saying she plans on beating Donald Trump.

BTW ... Elon Musk used the Politico piece to plug his own website -- retweeting the story to promote his own social media site.

Stories are sure to pour out in the next few days regarding Biden's decision ... but, it sounds like he made the choice with a tight-knit group.