Meghan Markle and Prince Harry are headed back across the pond temporarily ... but they're leaving at least one important family member behind.

Sources familiar with the situation tell TMZ ... Meghan's mom -- Doria Ragland -- isn't making the move with the couple despite her close relationship with her daughter and her son-in-law.

We're told Doria maintains a home near Los Angeles, and she has a life she's not ready to leave behind. Our sources say she will see the family ... though it's unclear where.

Our sources say Doria's active in the local yoga community ... and she has quite a few friends here as well.

Meghan and Harry's big move will change the family dynamic ... Doria often drove up to Montecito to visit them and her grandkids ... and she even had her own quarters in the massive home.

Worth noting ... our source says Meghan and Harry's move back to the UK is a temporary decision. When asked if it's just for 6 months or a year, our source said it's really more of a day-by-day decision.

As we reported ... Meghan and Harry are headed back to the UK just six years after stepping away from the Royal Family and shunning their royal duties.

The couple's children -- Prince Archie and Princess Lilibet -- are already enrolled in a school that starts next month.

Apparently, this didn't come as a surprise to King Charles ... who was informed by phone on Sunday that the move was in the works.