Cook, a leader in his field who founded Norton Rose's competition department nine years ago and built it up to four partners and 12 assistants in London and Brussels, said he had decided to make the move after meeting a Macfarlanes partner and old Oxford University friend at a Magdalen College reunion.
“He said 'if you ever get fed up at Norton Rose, consider us,'” said Cook.
He added: “I get more of a buzz out of creating and building a practice than managing an existing one.”
Norton Rose was expanding internationally in project finance, asset finance and banking, he said, fields in which UK and EU competition work is less central.
Cook will work alongside Macfarlanes' existing competition partner Jane Whittaker and they are seeking to recruit one or two assistants immediately.
Macfarlanes corporate partner Charles Martin said: “This is the first time we have hired a partner of this seniority – the first time we have taken a partner from the top 10 firms…” He said the move indicated that the firm's mind was not closed to the idea of a merger.
He stressed: “There is a myth that we are determined to remain medium-sized – we do not have a growth target.”
Cook will be replaced as head of competition at Norton Rose by Martin Coleman, whom Cook originally recruited and who returned to London from Norton's Brussels office six months ago.
David Lewis, Norton Rose senior partner, said: “Cook has done a very good job and we are sorry to see him leave.”