Sir Tom Scholar was fired this week - a move seen as part of a pledge by Ms Truss to change "Treasury orthodoxy".

Former cabinet secretaries Lord Butler and Lord O'Donnell criticised the decision, saying it compromised civil service independence.

The Treasury did not directly comment on the remarks.

Instead, it referred to a previous statement that thanked Sir Tom for his "dedicated service".

On Thursday, Sir Tom left as Treasury permanent secretary after six years, saying new Chancellor Kwasi Kwarteng wanted "new leadership".

Speaking to BBC Radio 4's The World This Weekend, Lord Butler said his "very unusual and very regrettable" departure reminded him of the US, where it is common for incoming governments to appoint their own officials.

"I think they are behaving improperly towards the civil service," he told the programme.

"A government wouldn't come in and on the first day sack the head of the her Majesty's defence forces, the chief of the defence staff," he added.

The crossbench peer, who was cabinet secretary - the UK's top civil servant - under Margaret Thatcher, Sir John Major and Sir Tony Blair, said the departure would prove disruptive.

"It'll weaken them, but it'll also corrupt our system, because one of those great advantages of having an independent, loyal civil service will be compromised."

'Abacus economics'

Lord O'Donnell, who was cabinet secretary to Sir Tony as well as successors Gordon Brown and David Cameron, said Ms Truss's first few days in government hadn't been "as respectful as one would like".

"Sacking someone with no notice for no apparent reason - someone held in high regard by chancellors of all political parties - is is no way to earn the respect of the Treasury and the civil service," he told Times Radio.

Throughout her campaign to be Tory leader, Ms Truss blamed "Treasury orthodoxy" for slow economic growth over recent years.

The new PM, a former Treasury minister, has accused her old department of promoting an "abacus economics" of "making sure that tax and spend add up," saying greater focus was required on promoting economic growth.

She has promised to boost the economy through "bold" tax cuts - a move that former chancellor and leadership rival Rishi Sunak has predicted will stoke further inflation.

Dave Penman, the boss of the FDA union that represents senior civil servants, accused Ms Truss of conducting an "ideological purge" of top officials.

Speaking on Saturday, former Tory Chancellor Philip Hammond said he was "disappointed but not surprised" by Sir Tom's ousting.

"I am a little bit alarmed at what has become a bit of a trend in British politics for incoming prime ministers to fire senior civil servants or at least to manoeuvre them out of the door," he added.

Asked to comment, the Treasury referred to the notice of Sir Tom's departure, which thanked him for his "dedicated service" and said a replacement would be appointed shortly.