Groom, 48, says: “I gave Lewis Silkin a year's notice under my partnership deed. I hoped we could have reached a compromise earlier, but unfortunately it did not happen.”
Groom joined Lewis Silkin in 1985 as a partner and within two years was in charge of advertising and marketing law.
But insiders say he was not happy working for a firm that did not have a specific advertising and marketing unit.
Instead he was attached to the litigation department, despite being regarded by his peers as the best advertising and marketing lawyer in the industry.
From this week, he will head Osborne Clarke's marketing and brand unit in London along with Tim Birt.
Birt says: “The fact that Lewis Silkin did not let Stephen leave before his year's notice was up is unusual, but I suppose it shows just how important they considered him to be.”