Three of five senior fund managers recruited by Deutsche Asset Management in the US from rival Invesco have turned the tables by filing lawsuits on their former employer, ratcheting up a legal fight that began with Invesco taking DeAM to court over the hires.

Stephen Johnson, former chief investment officer of fixed income at Invesco, and his colleagues James Guenther, director of credit research, and Kenneth Bowling, head of US fixed income, claim Invesco's insistence on a 12-month notice period is "unenforceable".