"Class-based bias" still exists in Westminster, according to the prime minister’s former director of communications.
Lee Cain said he had "lost count of the times I was branded a ‘bruiser’, ‘thuggish’ or even an ‘oik’ for the twin crimes of having a strong northern accent and shaved hair".
"My experiences in Westminster made it easy to see why young working-class women and men struggle to get ahead," he wrote in an article for the Spectator magazine.
Mr Cain, a former journalist who left Number 10 after a bitter power struggle in November, added: "When I first entered Downing Street I got the distinct feeling that some senior officials thought I should be content simply to be there.
"The clear message was: you’ve come a long way, don’t overreach now."
He said children from poorer households struggle to make it into "big professions" and when they do they "struggle to break through the ‘glass ceiling’."
"Britain’s most powerful jobs are still dominated by the tiny proportion (7%) who attended private school," he said. "No surprise that Fleet Street and Whitehall are among the worst offenders.
"It still matters what school tie you wore, it matters what accent you have and it matters what class you are. This is a disaster for working-class boys and girls."
Mr Cain said if the trend continues, "political parties and business leaders will find themselves ever more baffled and out of touch with the people they seek to lead".
In a bid to illustrate his point, he recalled discussions in government last summer over free school meals.
Ministers were coming under pressure from campaigners, including footballer Marcus Rashford, to extend meal vouchers to eligible pupils over the summer holidays.
Mr Cain admitted the government was "losing a PR war" with Rashford at that point and recounted attending a cabinet meeting with Boris Johnson and his top team.
"I asked the people running the country how many of them were ever eligible for free school meals and discovered I was the only person in the room who had been," Mr Cain said.
"Is that the reason the government got itself in such a terrible tangle over this emotive issue? No.
"But would it have helped if more senior politicians had had personal experience of food poverty? Undoubtedly."
However, Mr Cain praised the prime minister, saying he was a leader who "believes in social mobility" and has put "levelling up" at the "very centre of his premiership".
"Boris Johnson understands that the need for social mobility is now more urgent than ever, as we emerge from lockdown," he added.
Mr Cain followed Mr Johnson into Downing Street in 2019 after helping run his successful campaign for the Tory leadership.
He was a close ally of Dominic Cummings, Mr Johnson’s former chief adviser, having worked with him on the Vote Leave campaign for Brexit in 2016.
Mr Cummings left Number 10 himself around the same time as Mr Cain.