The actor, who has played detective Ronnie Brooks for eight series, said the character had become 'one of my best friends'.
He said: "It's been an absolute pleasure to inhabit Ronnie's Mac for as long as I have. Eight series is a wonderful achievement for everyone involved in the production.
"This has been one of the hardest decisions I have ever had to make. I hope one day to revisit him, but for now I'd like the opportunity to pursue other drama projects, which ITV are developing."
The final episode of Law And Order: UK is on ITV at 9pm next Wednesday (June 11), but studio bosses said it could make a comeback.
Director of drama commissioning Steve November said: "There may well come a time when we re-visit Law And Order: UK. For the moment, we'll be resting the series whilst we continue to refresh our drama slate."
Law And Order: UK, based on a popular US drama, was first shown in the UK in 2009.
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