Former top Home Office civil servant Sir Philip Rutnam has formally launched legal action against Priti Patel.
The department's ex-permanent secretary is claiming "constructive dismissal" in the claim submitted to an employment tribunal on Monday morning, civil servants' union the FDA confirmed.
Sir Philip quit his post in February amid allegations of bullying behaviour by Home Secretary Ms Patel.
The news of the latest move in his legal case comes as the Cabinet Office is expected to conclude an inquiry into claims that the minister clashed with senior officials and belittled colleagues.
Johnson silent over when he first knew of Priti Patel bullying allegations
Vanessa Feltz (left) sips tea with newly elected Witham MP Priti Patel during an event supporting Breast Cancer Care's Strawberry Tea fundraising campaign at the Houses of Parliament, Westminster, London.
Prime Minister David Cameron speaks with Witham MP Priti Patel in front of the Shah Sayyid Tomb in the Lodi Gardens in Delhi, India.
Prime Minister David Cameron and Witham MP Priti Patel, walk through Kolkata, India, where they visited the Howrah Bridge which was built by the Cleveland Bridge and Engineering Company.
Priti Patel who is the new Employment Minister, arrives at 10 Downing Street in London, as the PM puts the finishing touches to his new cabinet.
(left to right) Justice Secretary Michael Gove, Minister for Employment Priti Patel and Works and Pensions Secretary Iain Duncan Smith arrive in Downing Street to take part in the first Cabinet meeting since the Conservative Party won the General Election.
Indian Prime Minister Narendra Modi (centre) is flanked by Minister of State for the Foreign and Commonwealth Office, Hugo Swire (left) and Priti Patel (right) as he arrives at Heathrow Airport, London, for an official three day visit.
Employment minister Priti Patel launches a new government scheme to give extra careers advice to school pupils during a visit to Holy Trinity Catholic School in Birmingham.
(Left to right) John Whittingdale, Theresa Villiers, Michael Gove, Chris Grayling, Iain Duncan Smith and Priti Patel attend the launch of the Vote Leave campaign at the group's headquarters in central London.
EDITORIAL USE ONLY Minister for Employment Priti Patel tries her hand at painting with Matt Gray, Painting Skills Development Manager and Matt Pullen, CEO of AkzoNobel, as she officially opens AkzoNobel's Dulux Academy, the UK's first training facility dedicated to painters and decorators, Slough, Surrey.
Works and Pensions Secretary Stephen Crabb and Minister for Employment Priti Patel arrive for a Cabinet meeting in Downing Street, London.
Boris Johnson and Priti Patel meet workers at clothing and uniform manufacturers Simon Jersey in Accrington, Lancashire, during a visit as part of the Vote Leave EU referendum campaign.
Priti Patel speaks at a rally with Boris Johnson and Michael Gove (right) in Preston town centre, Lancashire, as part of the Vote Leave EU referendum campaign.
Michael Gove, Boris Johnson (centre) and Priti Patel pull pints of beer at the Old Chapel pub in Darwen in Lancashire, as part of the Vote Leave EU referendum campaign.
(left to right) Boris Johnson, Priti Patel and Michael Gove during a visit to Farmhouse Biscuits in Nelson, Lancashire, where they were campaigning on behalf of the Vote Leave EU referendum campaign.
Boris Johnson auctions a cow during a visit to a cattle auction in Clitheroe in Lancashire, where he along with Priti Patel and Michael Gove are campaigning on behalf of the Vote Leave EU referendum campaign.
Priti Patel MP visits Shree Sanatan Hindu Mandir Temple in Wembley, London, whilst out campaigning on behalf of the Vote Leave campaign.
Priti Patel arrives at White Waltham Airfield in Maidenhead, Berkshire, to meet veterans who will outline why they are voting to leave the EU.
(Left-right) Foreign Secretary Boris Johnson, International Development Secretary Priti Patel, Prime Minister Theresa May, Home Secretary Amber Rudd and Health Secretary Jeremy Hunt during Prime Minister's Questions in the House of Commons, London.
The Prince of Wales and the Duchess of Cornwall with International Development Secretary Priti Patel (right) during a reception and dinner for supporters of the British Asian Trust at Guildhall, London.
Priti Patel (left) with Boris Johnson's father Stanley Johnson and sister Rachel Johnson watch him speaking at a fringe event at the Conservative Party annual conference at the International Convention Centre, Birmingham. (Photo by Stefan Rousseau/PA Images via Getty Images)
Prime Minister Boris Johnson and Home Secretary Priti Patel on a walkabout with local police during a visit to North Road, Harbourne, Birmingham before announcing his plan to recruit an extra 20,000 police officers and an urgent review will take place of plans to make it easier for forces to use stop-and-search powers.
Discover More Like This
BACK TO SLIDE
In a statement, FDA general secretary Dave Penman said: "On 29 February 2020, Sir Philip Rutnam resigned as permanent secretary of the Home Office, indicating that he intended to pursue a claim of constructive dismissal.
"Following his resignation, the FDA instructed Gavin Mansfield QC, head of Littleton Chambers and employment law specialist, as counsel to advise Sir Philip, supported by Clive Howard, senior principal lawyer, employment and partnership at Slater and Gordon.
"This morning, Sir Philip, 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.
"Sir Philip will not be making any further comment at this time."
When Sir Philip quit his post earlier this year, he launched a blistering attack on Ms Patel, claiming he had been the target of a "vicious and orchestrated briefing campaign" which he accused her of orchestrating, and said the decision to resign was with "great regret after a career of 33 years".
In a statement released by his lawyers at that time, he said: "The Home Secretary categorically denied any involvement in this campaign to the Cabinet Office.
"I regret I do not believe her. She has not made the effort I would expect to dissociate herself from the comments."
He went on to claim that despite his efforts at reconciliation, Ms Patel "made no effort to engage with me to discuss this", adding: "I believe these events give me very strong grounds to claim constructive, unfair dismissal and I will be pursuing that claim in the courts."
Reports of feuding at the top of the Home Office prompted the head of the Civil Service, Cabinet Secretary Sir Mark Sedwill, to tell all civil servants a week earlier that advice they provide for ministers and "any debates" around it should remain "private".
One report suggested Ms Patel had tried to move Sir Philip from her department after they had a series of rows.
It was also claimed in various reports that the Home Secretary was distrusted by intelligence chiefs – which the Government denied.
Ms Patel expressed concern at the "false" claims, while allies described her as a "demanding" boss but not a bully.