Former top British civil servant Philip Rutnam has formally launched legal action against home secretary Priti Patel.
He is claiming "constructive dismissal" in the claim submitted to an employment tribunal, union the FDA confirmed.
Mr Rutnam quit the British Home Office in February amid allegations of bullying behaviour by Ms Patel.
It comes as the UK Cabinet Office is expected to conclude an inquiry into claims Ms Patel clashed with senior officials and belittled colleagues.
FDA general secretary Dave Penman said: "The FDA instructed Gavin Mansfield QC, head of Littleton Chambers and employment law specialist, as counsel to advise (Mr Rutnam), supported by Clive Howard, senior principal lawyer, employment and partnership at Slater and Gordon.
"(Mr Rutnam), with the support of his legal team and the FDA, submitted a claim to the employment tribunal for unfair (constructive) dismissal and whistleblowing against the home secretary."