The entire 15-strong team, including partner John Olsen and five lawyers, will go at the end of the month.
Olsen says he is moving to Field Fisher because intellectual property is a core practice there, unlike at SJ Berwin. "I am a full-range trade mark lawyer and I wanted to offer full-service trade mark law... so I am going to a firm which is the same as me."
Olsen's client base is believed to have a turnover of more than £2m a year.
Sources claim other business, including intellectual property litigation, may follow Olsen to Field Fisher.
SJ Berwin managing partner David Harrel says there are no plans to shed other parts of the business. "That's not the position as we see it... we weighed that up carefully when we decided this was something we had to do."
He says the decision was taken to shed the group because it did not fit with the firm's principal business of advising on corporate transactions.
"It deals with high volumes and we decided that we didn't want to be in the high-volume game," he says.
Mark Abell, head of intellectual property at Field Fisher, says the enlarged trade mark department will be the biggest one-stop shop in the UK.
SJ Berwin's Harrel says that there are no plans to shed other parts of the firm's business. "Like all firms, we are always looking at whether things fit and whether partners' expectations can be met by being somewhere other than with us."
Nils Montan, vice-president, senior intellectual property counsel at Time Warner Entertainment, says: "John Olsen is a leader in the trade mark field. This move is excellent news for his clients."