The chief executive of the civil service is preparing to step down amid a looming shake-up of Whitehall spearheaded by Dominic Cummings, Boris Johnson’s top adviser.

Sky News has learnt that Sir John Manzoni, who was knighted in the New Year honours list, has alerted senior colleagues that he intends to step down later this year.

A government source described Sir John’s retirement as “planned” and said he had been an important part of civil service reforms since 2014.

The news of Sir John’s departure is said to be unrelated to an embarrassing gaffe in late December which saw the private addresses of more than 1,000 honours recipients briefly posted online.

Nevertheless, the disclosure that he is drawing up plans to step down comes as the prime minister and Mr Cummings plot what has been described as a revolution in the way Whitehall is run.

There are roughly 400,000 members of the UK civil service, making it one of the largest single workforces in Britain.

In a controversial blog posted last month, Mr Cummings wrote that he wanted “assorted weirdos” to take on positions of influence within the government machinery.

Whitehall departments are also facing a shake-up in a post-Brexit reshuffle, with the Department for Exiting the EU about to be abolished and others under threat.

A former executive at the oil company BP, Sir John was recruited into Whitehall in February 2014 as chief executive of the Major Projects Authority.

He took over as chief executive of the civil service in October of that year and became permanent secretary at the Cabinet Office in 2015.

Sir John reports to Sir Mark Sedwill, the cabinet secretary.

He is credited by allies with having professionalised aspects of the civil service, including functions such as human resources.

Sir John also oversaw much of the planning in Whitehall for a possible no-deal Brexit.

Prior to joining the civil service, Sir John also served as president and chief executive of the Canadian natural resources company Talisman Energy.

Whitehall insiders tipped permanent secretaries including Alex Chisholm, who holds that post at the Department for Business, Energy and Industrial Strategy, as possible successors.

Others include his counterparts at the departments of health and Claire Moriarty, who holds the post at DEXEU, they added.

One source said that whoever succeeded Sir John was likely to hold the title of chief operating officer, rather than chief executive.

A Cabinet Office spokesman declined to comment.