"Opaque" dress codes practiced by those from more formal backgrounds are being used to judge candidates, with brown shoes a fashion faux-pas that many from poorer backgrounds may not be aware of, said the study.
"Managers often select candidates for client-facing jobs who fit the traditional image of an investment banker and display polish," it said.
"For example, some senior investment bankers still deem it unacceptable for men to wear brown shoes with a business suit."
One interviewee from a non-privileged background explained he was rejected despite being told that he had interviewed well.
"He said 'you're clearly quite sharp, but you're not quite the fit for (this bank), you're not polished enough'," he said of the explanation given for his rejection.
Alan Milburn, chair of the Social Mobility Commission, said it was "shocking, for example, that some investment bank managers still judge candidates on whether they wear brown shoes with a suit, rather on than their skills and potential".
"Bright working-class kids are being systematically locked out of top jobs in investment banking because they may not attend a small handful of elite universities or understand arcane culture rules," he added.
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