The “shameful” treatment of Britain’s ousted ambassador to the US marks the culmination of a long-running campaign to abuse and scapegoat government officials, senior civil servants warn today.

They say that Sir Kim Darroch’s resignation last week was the latest episode in a “wider culture of abusing civil servants which has been allowed to develop over the last few years”.

Darroch quit his post after confidential emails, in which he described Donald Trump’s administration as “clumsy and inept”, were leaked to a Sunday newspaper. A police investigation is under way into the original leak, and last night further cables from Darroch were published. There are concerns that the leak was designed to have a chilling effect on the frank advice officials are willing to give ministers.

In a letter to the Observer, the heads of the Prospect and FDA unions, which between them represent tens of thousands of Britain’s civil servants, said Darroch’s removal followed briefings against officials and “sustained attacks”.

“No group has done more to keep the country running than the civil service,” the general secretaries write. “As a reward for this, civil servants have been subjected to sustained attacks from across the political spectrum. In recent months we have seen a malicious briefing campaign against individual civil servants involved in Brexit negotiations, attacks on Treasury officials when their forecasts warn of the dangers of no deal and the disgraceful attempt to blame civil servants for the national security leak involving Huawei. The culmination of this has been the disgraceful and unprecedented treatment of Sir Kim Darroch this week. This episode is part of a wider culture of abusing civil servants which has been allowed to develop over the last few years.”

Sir Malcolm Rifkind, the former foreign secretary, targets Boris Johnson for refusing to back Darroch in the face of Trump’s criticisms and calls on Theresa May to appoint the next ambassador.

“That choice should be made swiftly by the prime minister in consultation with the two candidates to succeed her,” he writes in the Observer. “David Cameron appointed the new British ambassador to Paris a few days before he resigned, with minimal consultation. There is no reason why Theresa May, consulting with Hunt and Johnson, cannot choose a successor in the next two weeks.”

Three former cabinet secretaries – Lord O’Donnell, Lord Butler and Lord Armstrong – have added their voices to the growing chorus of concern.

Butler said it would be “an act of self-harm to compromise our non-political civil service”, while Armstrong said the principles of political impartiality had given the UK “one of the least corrupt and most dependable public services in the world”. He added: “They are not an outmoded relic of former times; they are a necessary condition of good government.”

O’Donnell said: “One of the reasons our civil service is officially rated the best in the world is adherence to the values of objectivity, integrity, honesty and impartiality. Politicians will get the best out of the civil service if they encourage honest, robust advice and in return show respect for their commitment to honesty and impartiality. This applies to home civil servants and our diplomats.”

Darrroch has been taken out by an act of partisan vandalism in the hope of replacing him with a political appointee more to Brexiters’ taste Lord Ricketts

Sir Nigel Sheinwald, one of Darroch’s predecessors as the UK ambassador in Washington, said the leak of emails could have a wider impact on the advice officials are willing to give. “Kim Darroch may have been the immediate target of the leak, but I suspect whoever was behind it has in mind a wider target of our system of public service and wanted to influence the way the next government handled the civil service, and was perhaps trying to influence the succession of Kim Darroch. This undermining of the public service has been going on for years and is clearly going to be a fundamental problem in the period ahead.”

Lord Ricketts, the former chief civil servant in the Foreign Office, said Whitehall had been “badly let down” in the wake of the “Brexit tornado [that] has ripped through British politics”.

“Civil servants worked heroically to help a dysfunctional cabinet shape and deliver an agreement with the EU,” he said. “Sir Ivan Rogers [the former UK ambassador to the EU] and Oliver Robbins both faced sustained political attacks on their integrity with precious little support from ministers. Now Sir Kim Darroch has been taken out by an act of partisan vandalism, presumably in the hope of replacing him with a political appointee more to Brexiters’ taste. This corrosive environment is doing real damage to an organisation that has served Britain well for over a century.”

Lord Jay, a former ambassador to Paris who oversees the Lords EU select committee, said: “Civil servants and diplomats must be able to offer advice and to report the views of others knowing their confidence will be respected. Ministers of course decide policy, but policy will be less well based and the quality of governance will suffer – from which we will all be the losers – if civil servants and diplomats are or feel muzzled.”

On Saturday night further cables emerged in which Darroch described Trump’s decision in 2018 to abandon the Iran nuclear deal.

President Obama had hailed the deal as a “historic understanding” in 2015, when Iran had agreed to reduce its uranium stockpile and allow inspections of its nuclear facilities, in exchange for the lifting of US and EU sanctions.

The memo from Darroch to Boris Johnson was written the day after the then foreign secretary had flown to Washington on 7 May 2018 to try to save the deal – unsuccessfully.

Darroch praised Johnson for obtaining “exceptional access”, meeting US secretary of state Mike Pompeo, Trump’s national security adviser, John Bolton, his chief of staff, John Kelly, as well as Ivanka Trump and Trump’s son-in-law Jared Kushner. “The outcome illustrated the paradox of this White House: you got exceptional access, seeing everyone short of the president; but on the substance, the administration is set upon an act of diplomatic vandalism, seemingly for ideological and personality reasons – it was Obama’s deal,” Darroch’s memo said, according to the Mail on Sunday.

“Moreover, they can’t articulate any ‘day-after’ strategy; and contacts with State Department this morning suggest no sort of plan for reaching out to partners and allies, whether in Europe or the region.”