Despite Mrs May's decision to keep Cabinet “big beasts” such as Philip Hammond and Boris Johnson in place in this month's reshuffle, the report by the Institute for Government found that 71 per cent of ministers - 85 out of 122 - are new to their jobs since last year's general election. 

At the crucial Department for Exiting the EU, only two ministers have stayed in place since July 2016, while every minister in the Cabinet Office and three-quarters of those in the Ministry of Justice were replaced in the January 2018 reshuffle.

The Justice and Culture departments are each on their sixth secretary of state since 2010, while there have been five work and pensions secretaries since 2016. 

Meanwhile, the IFG's annual Whitehall Monitor found that Brexit is driving an increase in civil service staffing after years of cutbacks. 

Since the EU referendum in 2016, more than 8,000 additional civil servants have been taken on, with the upward trend “looking set to continue”, as the Home Office recruits 1,200 immigration caseworkers and 300 border staff. 

Many of the recruits are in the new Brexit-based Departments for Exiting the EU and International Trade, but others with high exposure to Brexit workstreams, like Environment and Business, were also increasing staff numbers. 

“As Theresa May's Government enters 2018, Brexit is rightly absorbing significant effort and is the main reason why the civil service has grown in the past year, after years of shrinking,” said IFG director Bronwen Maddox. 

“But the Government has made commitments to voters on public services, productivity, social mobility and major projects. If it fails to meet their expectations, it risks further undermining confidence in government.” 

The report found that “preparations for Brexit have been disrupted by the election, by turnover in personnel and by difficulties in parliamentary management”. 

While the need to maintain a delicate balance of Brexit views within the Cabinet deprived Mrs May of a “free hand” in choosing her top team, her reshuffles have unleashed considerable upheaval in the lower ministerial ranks, said the report, noting pointedly that this is “where a lot of government business gets done”. 

Stability in ministerial posts is generally welcome, as “it is better to avoid moving ministers just as they are getting to grips with their department”, said the report. 

But 44 per cent of all ministers appointed after Mrs May took office in 2016 were new to their jobs, along with 38.5 per cent of those in post after the January 2018 reshuffle. 

The growth in the size of the civil service has been greater among top mandarins than in the lower ranks, the report found. 

Numbers in more senior grades – usually based in London – have been increasing since 2012 and are now at a higher point than at the start of austerity in 2010. 

Only at the most junior levels, including prison officers and JobCentre staff – more likely to be located in the English regions - are numbers continuing to fall. 

The report found “little recent progress” in numbers of senior civil servants with disabilities or ethnic minority backgrounds, while the percentage of women decreases as you ascend the Whitehall pecking order. 

Of those appointed to the highest departmental rank of permanent secretary in 2017, “as many were men with the surname Rycroft as were women - two in each case”, the report noted. 

The IFG found that Government departments were “opaque” in their responses to Freedom of Information inquiries, withholding information in 52 per cent of cases in the third quarter of 2017, compared to 39 per cent in 2010. 

David Davis's DExEU had “a particularly poor record” as the department least likely to release information.