Civil servants have been told they will have to “do things differently” by their new boss, the cabinet secretary, on his first day in the job.

Sir Chris Wormald has promised a “rewiring of the way the government works”, in a message to half a million civil servants.

Sir Chris, who previously ran the departments for health and education, has taken charge of the civil service less than two weeks after Sir Keir Starmer claimed “too many” officials were “comfortable in the tepid bath of managed decline”.

The FDA union, which represents senior civil servants, has accused the new government of taking an “astonishing” and “really damaging” approach.

Labour officials entering government for the first time have privately complained that the Whitehall machine is bloated and outdated.

But FDA general secretary Dave Penman said civil servants “feel a sense of betrayal” by the prime minister.

In his all-staff message, Sir Chris said he was “deeply honoured” to take up his new job, adding it was his role “to support the prime minister and government to deliver for the country”.

“The prime minister has been clear that he wants a re-wiring of the way the government works to deliver his recently announced Plan for Change.

“This will require all of us to do things differently – from working much more effectively across departments to taking advantage of the major opportunities technology provides.

“Throughout my career, I have seen the civil service adapt and change to achieve incredible things for the people of the United Kingdom. I know that together we will relish the opportunities the coming years will bring.”

Sir Chris, who is 56, joined the civil service in the Department for Education in 1991, and later headed that department and the Department of Health and Social Care during the Covid pandemic.

He was the most experienced candidate on the final four-person shortlist for the cabinet secretary job.

When his appointment was announced earlier this month, he was praised by former health secretaries Sir Sajid Javid, who called him “brilliant, and Matt Hancock, who described him as a “natural reformer”.

Dominic Cummings, the former chief adviser to Boris Johnson during the pandemic, took a different view, saying the appointment was a sign “the Westminster system is totally determined to resist any change and will continue all the things of the past 20 years that have driven us into crisis”.

Sir Chris has replaced Simon Case, who stepped down after just over four years as cabinet secretary on health grounds. He has been undergoing medical treatment for a neurological condition for the past 18 months.

Case had at times been a controversial figure, facing scrutiny particularly over his role in the Downing Street parties saga and over messages he exchanged with ministers during the pandemic.

Last week, a government source told the BBC that more than 10,000 civil servants’ jobs could be cut as part of Labour’s push for 5% savings across departments.

Headcount in the civil service topped 513,000 this year, a 33% increase on 2016 and the eighth year in a row that the total has risen, according to the Institute for Government.

Ministers recognised civil service numbers “have gone up and up, and in reality that is not going to be able to continue,” the source said.