Border Force chief will win his case because of your attacks, May is told

May's lawyers said the public comments broke rules on handling disciplinary action

The Home Secretary, Theresa May, faces fresh embarrassment over the borders fiasco after being told by her lawyers that the civil servant at the centre of the storm will win his constructive dismissal case against the department.

Their conclusion means Brodie Clark, who earned £130,000 a year as head of the UK Border Force, is likely to receive a hefty pay-off reflecting his 40 years of service in Whitehall.

He and two colleagues were suspended last week after revelations that biometric and anti-terrorism checks were secretly eased over the summer at ports.

Ms May pinned the blame squarely on him, insisting she knew nothing about the decision. Mr Clark quit on Tuesday, complaining that he had faced such a campaign of vilification that he could not get a fair hearing.

Accusing Ms May of making misleading remarks about him, he said he was resigning to launch a constructive dismissal case.

The Home Secretary's lawyers advised her Mr Clark looks certain to win as the public comments – and private briefing – about him broke Whitehall rules on handling disciplinary action.

The hostile briefing against Mr Clark dismayed colleagues within the Home Office and other departments.

One Downing Street insider told The Independent: "It was an aggressive operation that backfired." Mr Clark was described by Home Office sources as a "rogue civil servant" and it was suggested he could even be prosecuted.

His union, the First Division Association, said Ms May had spent two days "traducing him and damning him".

Alan Johnson, the former Home Secretary, told MPs: "The treatment of Brodie Clark, who I know and respect and admire, has been reprehensible."

Mr Clark had maintained his silence until he resigned and is now preparing to set out his case when he appears before MPs next Tuesday.

The controversy erupted last week when Ms May accused him of covertly approving the relaxation of border checks on new arrivals from outside the European Economic Area (EEA) over the summer. She said she had authorised a pilot trial into easing checks on some EEA arrivals, notably children, and switching resources into higher-priority groups of passengers – but not its extension to other travellers.

The furore dominated Westminster for a third day yesterday, with Ed Miliband lambasting ministers' "shambolic" handling of the issue at Prime Minister's Question Time. Yvette Cooper, the shadow Home Secretary, said staff cuts forced immigration officials to reduce checks.

She produced an internal UK Border Agency email showing staff could authorise back-up at border controls only if checks had already been reduced.

Ms Cooper said: "We are seeing now that the scale of these cuts is putting pressure on the Border Agency."

Ms May said she "stood by every word" she had previously said and mocked Labour's immigration record.

She also pointed out it was Rob Whiteman, the UK Border Agency chief executive, who suspended Mr Clark – and she said he backed her version of events.

Damian Green, the Immigration minister, denied speculation that he and not Ms May had authorised extensions to the pilot scheme.