UBS has sued Jefferies after the head of healthcare banking at the Swiss group left with at least 34 colleagues to join the mid-market investment bank he reportedly described as "low quality" eight years ago.

UBS has taken action against Jefferies, Benjamin Lorello, head of the healthcare group and a vice-chairman of investment banking and Sage Kelly, a managing director, according to documents filed at the Supreme Court of the State of New York on June 22.