The Competition and Markets Authority (CMA) found both supermarkets breached a total of 65 competitive land agreements between 2011 and 2019.
Morrisons reportedly breached agreements 55 times, while M&S breached 10 agreements.
The CMA said Morrisons had the poorest compliance record with the order that the CMA has seen to date.
Although 14 of these restrictions have ended, there are an outstanding 41 restrictions that Morrisons has agreed to address.
Likewise, five of M&S’s restrictions have ended and it has agreed to address the remaining five.
Adam Land, senior director of remedies business and financial analysis at the CMA said: ”At a time when the weekly shop is a source of financial pressure for many families, it’s crucial that competition between supermarkets is working well to help people get the best deals they can.
“These restrictive agreements by our leading retailers are unlawful. There can be no excuses for non-compliance with an order made in 2010, especially when we know the positive impact for shoppers of new stores on the high street. “
The competition regulator has now received pledges from both groups to cease using unlawful land agreements.