Dave Penman is working overtime. The general secretary of the FDA union, which represents senior civil servants, says he’s taken more calls from worried members in the past week than in the previous 20 years, on the vexed issue of loyalty.

That rose to fever pitch on the day Kim Darroch resigned as UK ambassador to the US. But the pressure on civil servants has been building over the past few months, including attacks on the government’s chief negotiator, Olly Robbins, to a level where Penman says they now feel under attack from all sides. He describes the situation as “extraordinary”. “We have seen what is essentially a plot to oust one of the most senior public servants in the country,” he says.

Dave Penman, general secretary FDA union Photograph: Jane Dudman/Jane Dudman The Guardian

Diplomats and former diplomats have responded furiously to Darroch’s resignation. Former diplomat Jennifer Cassidy, now a lecturer at Oxford University, tweeted that “what we are seeing frightening and dangerous in equal measure”.

Former senior civil servant at HM Treasury Jill Rutter agrees. “The atmosphere around Brexit means raising practical objections is interpreted as hostility to the entire project … Simply asking the questions is seen as evidence that the civil service is not impartial on the project,” she wrote in a timely pamphlet on civil service impartiality, published on the day of Darroch’s resignation.

Rutter, now programme director at the Institute for Government thinktank, highlights the decision-making paralysis across Whitehall created by the cabinet’s chasm-like fissure over the “right sort of Brexit”. Civil servants, whatever their own political persuasion, work for the government of the day.

The row over Darroch’s leaked comments about the US president, and the shock of his departures across Whitehall, shows the impact when civil service confidentiality is breached. But evidence has been in plain sight over the past few years of the brutal impact of Brexit fervour on the 400,000 public servants who work for the UK civil service.

Many politicians come into government suspicious of the civil service. The shadow of Yes Minister’s Sir Humphrey falls long and deep. If a politician of any colour feels their cherished policies are going nowhere, it’s easy to characterise officials as progress-blocking, self-serving stuffed shirts.

It happened under the coalition government, as David Normington, former civil service commissioner, notes. Between 2010 and 2015, the then minister for the Cabinet Office “was frequently at odds with, and publicly critical of, the senior civil service. Significantly he was not reined in by the then prime minister.”

Quite so. In fact, David Cameron himself attacked the civil service, while Lord Kerslake, head of the civil service for two turbulent years, was the subject of almost constant off-the-record critical briefings – so much so that when he underwent a major back operation in the summer of 2014, he tweeted about a “pretty big knife in the back!”. When he stepped down that September, Kerslake said “noises off” – briefings against civil servants – had been the most damaging thing to civil service morale.

But there’s a big and frightening difference between the treatment of Kerslake and what has been going on since the referendum in 2016. Kerslake received only verbal abuse, however unpleasant. When Jon Thompson, the head of at HM Revenue and Customs, gave his considered, professional advice that the the post-Brexit customs option preferred by Brexiters would cost up to £20bn, he received death threats so serious that he had to change his route to work.

“We have had to literally change how I travel and what my personal security is. We have had two death threats investigated by the Metropolitan police for speaking truth unto power about Brexit,” he said.

Robbins has not revealed whether he, too, has received death threats. But, as Rutter points out, Brexiters’ distrust in the civil service reached its apogee in September 2018, when MP Richard Drax asked Robbins whether he believed that leaving the EU is a good thing. “You personally, in your heart, are you right behind us in every single sense of that?”

Robbins batted that question away – and is now departing the service altogether. Kerslake says the frequent attacks on Robbins and other senior civil servants are pernicious. He wants a public campaign to recognise and celebrate the enormous importance of having an impartial civil service. That’s unlikely in an atmosphere where Nigel Farage appears to be calling for the removal – really? – of all members of the civil service and the military who don’t “believe” in Brexit. It’s a fundamental misunderstanding of the role of an impartial civil service, argues an increasingly despairing and furious Penman.

Civil servants, bound by the civil service code of neutrality, cannot speak up for themselves. There are arguments that these times are so turbulent that rules should be overridden. In the meantime, Penman is trying to articulate the value of an impartial civil service at a time when, he says, the tectonic plates are shifting in how many civil servants will look at their relationship with ministers.