Laura Dern is joining the cast of “The White Lotus” … a major development that took very little time to happen after Helena Bonham Carter‘s abrupt departure last week.

A source with knowledge tells TMZ … Dern is not playing the same character as Helena … that role has been scrapped — and a new one is being crafted amid production.

As we reported … Helena left the show all of a sudden …. with filming starting about a week ago in France.

In a statement, a rep for HBO told TMZ … “With filming just underway on season four of ‘The White Lotus,’ it had become apparent that the character which Mike White created for Helena Bonham Carter did not align once on set.” The exact circumstances of her exit remain a mystery … with production lips sealed tight.

Laura is one of HBO’s darlings, starring in several award-winning series, including “Big Little Lies,” “Recount” and “Enlightened.” She and Mike have also worked together before on “Enlightened” and “Year of the Dog.”