The Civil Service union has welcomed the tone of Labour’s comments to its members in Whitehall and beyond, but warned the Government faces difficult decisions on pay across the public sector.

In an address to civil servants on Monday, the Prime Minister praised the “spirit” of the workforce and said it would “define the success of this government”.

Dave Penman, general secretary of the FDA union, welcomed Sir Keir Starmer’s comments, as well as a reassuring message by Education Secretary Bridget Phillipson to staff in her department.

One of the first things (the Government) are going to have to tackle is what they are doing about pay across the public sector, not just the Civil Service. So there is going to be big challenges for them

Mr Penman said he expects Labour to focus on “asking for more with the same rather than asking for the same with less” from the Civil Service, in contrast to the former Conservative government’s plan to reduce the workforce by 70,000 posts.

But he cautioned that the reality of being in government meant ministers faced difficult decisions on pay that would need to be addressed with urgency.

Mr Penman said: “One of the first things they are going to have to tackle is what they are doing about pay across the public sector, not just the Civil Service.

“So there is going to be big challenges for them. When in power, there needs to be real decisions rather than kind of throwaway remarks, or trying to take an opposite view of whatever the Conservatives did when Labour were in opposition.

“These things come at a cost and it’s a case of whether you have the ability to deal with this.”

Mr Penman said Sir Keir’s commitment to “respect and collaboration” with civil servants was in stark contrast to the approach of some Conservative ministers who he said actively undermined such an approach.

Providing an insight into perspectives in Whitehall on the former Tory government, Mr Penman said: “Most of the time, we would often get stuff from members saying ‘I’m tired of being thanked in private and criticised in public by ministers’.

“The ministers knew they relied on the Civil Service and were very respectful of the Civil Service, but they would pander to the rhetoric that it was a convenient punchbag, whether it was about home working or accusations about being pro-remain.”

When ministers get frustrated that things are not happening quickly - sometimes justifiably, sometimes unjustifiably - they are going to blame the Civil Service

Mr Penman criticised veteran Conservative Sir Jacob Rees-Mogg as “crass and condescending” when as a minister he left notes on civil servants’ desks in an effort to encourage them to limit homeworking.

Former home secretary Suella Braverman was also heavily criticised for an email in her name sent last year which accused civil servants of blocking efforts to stop small boat crossings.

Mr Penman said ministers “indulging in a culture war” showed the Civil Service was at the time seen as “a convenient tool to use” rather than ministers opting to “motivate and inspire” staff to help improve services.

But he praised Ms Braverman’s successor James Cleverly, who he said reassured civil servants that “if we disagree in private, I will defend you in public”.

“That’s quite a brave thing, pointedly criticising the approach of his predecessor,” Mr Penman added.

He said that while Labour had made a positive start, there would inevitably be challenges ahead for the relationships between ministers and civil servants.

The approach of a new government would always be “different at the beginning” but internal issues would emerge, Mr Penman said.

He added: “When ministers get frustrated that things are not happening quickly – sometimes justifiably, sometimes unjustifiably – they are going to blame the Civil Service.

I have no doubt that as a government they will make mistakes. As a government, there will be better and worse individuals

“You are going to get bad ministers. You are going to get bullies. They are going to have to make difficult decisions quickly.”

Mr Penman said Labour’s promise of “ethical government”, including a pledge to establish a ethics and integrity commission to ensure “ministers are held to the highest standards”, showed Labour “just doing the basics – what should be unexceptional”.

But again, he warned there would be problems ahead.

Mr Penman said: “I have no doubt that as a government they will make mistakes.

“As a government, there will be better and worse individuals. Also, they have got some tough choices to make on things like pay.”