Grayling, and its consumer and tech division Atomic, won the business in March, after M&S announced in December that it had ended its five-year relationship with Unity.
A spokeswoman for the retailer said: "Grayling picked up a couple of additional national projects with us after we parted company with Unity last year."
She added that Grayling was made aware its focus would shift to regional work after the Spend it Well campaign was completed.
Sarah Scholefield, Grayling UK and Ireland CEO, echoed this, telling PRWeek: "These were fixed-term projects which are now completed, and we are delighted to continue our core and significant regional work with M&S."
The spokeswoman said Grayling had "a big regional remit", which it has held since 2013, with PR for this work being done by the agency's offices in Leeds, Birmingham, Edinburgh, Southampton and Bristol.
The agency's remit includes consumer PR, work with stores, and crisis management on a local level, the retailer said.
Major PR shake-up at M&S
Earlier this year, M&S also severed ties with WPP-owned agency Finsbury, which had handled its corporate and City PR for four years, amid a major PR shake-up at the retailer.
The spokeswoman said at the time that Finsbury would be the last agency to do such work with the retailer, adding that much of Finsbury's duties would instead be handled by the company's in-house PR team.
Elsewhere, M&S continues to work with WH PR in the Republic of Ireland and ASG PR in Northern Ireland.