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His remarks have been condemned by Labour shadow minister John Ashworth who called on Mr Letwin – now a Cabinet Office minister – to apologise.

He poured scorn on claims the unrest was the product of urban deprivation. He also dismissed proposals by ministers to foster a new middle class of black entrepreneurs, saying they would simply set up in the “disco and drug trade”.

They show that Mr Letwin – then an adviser in Margaret Thatcher’s No 10 policy unit – blamed “bad moral attitudes” for the major disturbances which broke out in mainly black inner city areas.

His comments are revealed in files released by the National Archives which include a host of revelations about the workings of government in the 1980s.

PC Keith Blakelock was stabbed to death during the unrest

He said: “These offensive remarks are very concerning, particularly given his very senior position in the current Government as David Cameron’s policy supremo.

“I’m sure Mr Letwin will want to clarify these remarks and apologise as soon as possible.”

Labour MP David Lammy who grew up near the Broadwater Farm estate in Tottenham, north London, said the comments were “breathtaking”.

Mr Lammy said: “It had nothing to do with moral bankruptcy and everything to do with social decay and the appalling relations between black youths and the police.”

The riots of 1985 included serious unrest in Handsworth, Birmingham, in Brixton, south London and at Broadwater Farm in Tottenham, where PC Keith Blakelock was stabbed to death.

The troubles were blamed on a combination of high unemployment, slum housing, poor education and distrust between young black people and the police.

But in a memorandum, written with the future Tory MP Hartley Booth, Mr Letwin said the riots came down to “individual characters and attitudes”.

They wrote: “The root of social malaise is not poor housing, youth ‘alienation’ or the lack of a middle class. Lower-class unemployed white people lived for years in appalling slums without a breakdown of public order on anything like the present scale.

“Riots, criminality and social disintegration are caused solely by individual characters and attitudes. So long as bad moral attitudes remain, all efforts to improve inner cities will founder.”