Civil servants with the "right accent" have a better chance of climbing the ladder than those from vastly under-represented disadvantaged backgrounds, a new report has found.
Nearly 72% of senior officials had privileged upbringings, with a quarter of those in the top jobs having attended an independent school, the damning report from the Social Mobility Commission revealed.
Analysis of more than 300,000 civil servants showed that just 18% of the senior Civil Service came from a disadvantaged background.
Staff who were promoted were likely to have been privileged, have the right accent or "received pronunciation", an emotionally detached and understated manner, and an "intellectual approach" to culture and politics.
Dr Sam Friedman, incoming professor of sociology at the London School of Economics, who conducted the report, discovered that those who get to the top "use existing networks".
"The right accent and a ‘studied neutrality’ seem to win through at every stage of their career. Even at the lower end of the profession, progress is thwarted for those who don’t know the rule," the report stated.
It also showed that black civil servants routinely battle "classed stereotypes of blackness that are both offensive and bear no resemblance to their actual lives and experiences".
One civil servant interviewed for the report said: "So, my sense from quite early on was that there was a secret code as to how to get on.
"There were these folk that worked in the Treasury, had done certain things… they knew about ‘the velvet drainpipe’, as you hear it described. The way up and through. And they’d clearly done it, and they had a language to speak about it."
Following the findings, the Commission recommended that class should be a "protected characteristic" among other factors such as race, gender, disability, age, and religion.
Steven Cooper, interim co-chair of the Social Mobility Commission, said: "Civil servants from disadvantaged backgrounds are significantly under-represented in the organisation, and even if they do ‘get in’ they can struggle to ‘get on’."
The report said that while the Civil Service has been praised for fair recruitment and publishing workforce data, "socio-economic diversity has rarely been scrutinised, and not for several decades".
But the organisation "is acutely aware that it needs to do better" and had developed an action plan, the report added.
The Commission also recommends that the virtual working of parliament should become permanent to enable MPs and ministers, with Civil Service hubs, to be based outside London.
The data revealed showed strong regional divides among staff, with just 22% of London-based civil servants being from a low socio-economic background, compared with 48% in the North East.
There was also a split across departments, with people from working-class backgrounds making up just 12% of those working at the Treasury, compared with 45% at the Department for Work and Pensions.
Dr Friedman said: "An important part of progressing through the labyrinth of the Civil Service is mastering the unwritten rules; what jobs to take, where to work, how to negotiate opportunities, and above all how to behave.
"And, strikingly, it is those from privileged backgrounds who hold the upper hand in unpicking these hidden rules."
Civil Service social mobility champion Bernadette Kelly, who is also permanent secretary at the Department for Transport, said: "We are strongly committed to driving progress on socio-economic diversity in the Civil Service.
"No one should be held back from achieving their full potential because they come from a less privileged background. We are already acting on many of the findings and recommendations in this report – for example, we are extending apprenticeships and moving hundreds of senior Civil Service jobs out of London.
"I hope it will help us to focus our efforts on the actions that will have the most impact."