I’ve worked in the civil service for most of my adult life, including a stint working in the no-deal Brexit planning bunker Operation Yellowhammer back when we thought we’d crash out of the EU in March. But with a so-called war cabinet “turbo-charging” preparations in the 90 days before we all lurch into a Halloween no-deal meltdown, the UK body politic is in something of a state of shock. And there are no grownups in sight.

After the night of the blond knives, “funereal” is the best word to describe the atmosphere in my patch of Whitehall. Boris Johnson has constructed his cabinet out of the most radioactive remnants of the leave campaign, starting with the installation of the leave campaign witch doctor, Dominic Cummings, at the heart of the No 10 policy machine. What happened? Was Moriarty unavailable? Had the burning Eye of Sauron already turned down the job because it wanted to “spend more time with the family”? It’s difficult to imagine a more alarming appointment – but it shouldn’t have been a surprise.

The most illuminating and disturbing message was delivered by the prime minister himself in that brash, electioneering speech on the steps of No 10. My Whitehall colleagues, not least those crammed up against the railings of Downing Street to watch him, could scarcely have missed its polarising subtext. As former Treasury mandarin Jill Rutter wrote last week, that message is simple: Johnson will simply ignore or disavow those who do not believe in his optimism project.

Well, so what? Surely any new prime minister is entitled to surround himself with kindred souls as he swashes his buckles in pursuit of his Brexit “moonshot”. Why does it matter if the nation’s civil servants feel threatened – if anything, isn’t that what these pusillanimous, backsliding Whitehall gloomsters need to corral them back into the one, true church?

‘The subtext to Boris Johnson’s brash speech on the steps of No 10 was clear: he will simply ignore or disavow those who do not believe in his optimism project.’ Photograph: Paul Marriott/REX/Shutterstock

No. It matters because this “optimism on steroids” masks dangerous side-effects that present almost as many risks as Brexit itself. There are a number of core values that every civil servant signs up for – yet each of these is under threat, with potentially serious consequences.

First, objectivity: civil servants are duty-bound to base their advice on rigorous analysis of the best evidence available, whether it’s no-deal economic analysis or how best to run an NHS hospital. Brexiteers, however, including cabinet members, have no problem throwing accusations of bias and apostasy at senior civil servants when evidence inconveniences policy.

That matters because while throwing us publicly under the bus is bad enough, what’s worse is the prospect of civil servants – fearing defenestration or public shaming by ministers – ignoring important evidence, which could potentially put lives at risk.

What would that look like? Forget Brexit for a minute. Only last week, the public accounts committee censured the Home Office for its “unhealthy good news culture”, which led to over-optimistic officials ignoring evidence that a new nationwide communication system for the emergency services (now horrendously delayed) was in fact undeliverable. Expect this corrosive optimism to deliver more of the same.

Second, honesty. At prime minister’s questions, the Labour MP Angela Eagle said “one of the principles of public life is honesty” – and asked if Johnson had always been honest in his political career. What’s ominous is not that he didn’t answer the question; it’s that no one expected him to. That matters because beyond Brexit lie a range of other wickedly disruptive challenges, including climatic, technological and demographic changes. Those challenges can only be overcome by being honest about the risks, benefits and costs. The alternative is an increasingly partisan US-style civil service that falls into dangerous groupthink.

Finally, integrity. The UK’s civil service was rated No 1 in the world a few months ago by a respected international index. Unfortunately, because civil servants take instructions from their ministers, that’s no guarantee against disaster. Also, that assessment of our civil service masks important weaknesses. It turns out that the UK’s lowest score in the index (barely average compared with the civil service machinery in other countries) was for integrity – defined as the commitment to put public service above personal interest. This is where Brexit could hit the civil service hardest, and where we come back to Cummings.

Why has his appointment provoked such alarm? He has declared war on Whitehall. He is a shock doctrine aficionado who has seen in Brexit a once-in-a-lifetime opportunity to ram through policy ideas that couldn’t be implemented at any other time. Some of those ideas may be good ones. Others, like allowing an increased politicisation of the civil service, are not. The problem with most of them is that they are untested; if they go wrong they would, according to the civil service union chief, David Penman, “impact upon the delivery of public services to millions of citizens”.

This week we saw a growing number of MPs voicing their concern that he represents a threat to democracy itself. That’s unprecedented – and we should heed their warning. This is not about opposing the idea of confidence or optimism – but the normalisation of a myopic ruthlessness and a corrupting desire to go for broke, to win by any means necessary.

• The civil servant works in a Whitehall department and was part of Operation Yellowhammer