Six primary care providers at a single Mass General Brigham practice are moving to rival Beth Israel Lahey Health in January, raising to at least 18 the total number of such defections from the state’s largest health care system to the second largest in less than a year.

The move, the latest development in the two institutions’ fierce rivalry, is expected to cost MGB millions of dollars in revenue. It also represents another symbolic blow to the health system, which has been under fire from some of its own primary care physicians .

“It’s relatively unusual for people to change affiliations at all, and particularly from the dominant market player to the second-biggest market player,” said Paul Levy, who served as chief executive of Beth Israel Deaconess Medical Center from 2002 to

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