Brown shoes and a loud tie could deny a bright young candidate a job in banking, according to a report.
The government-backed Social Mobility Commission said that applicants from poorer backgrounds are not familiar with "opaque" codes of conduct that privileged candidates are comfortable with.
The report noted that 7% of children in the UK attend fee-paying schools, although 34% of new investment bankers had attended a public school.
It added that banks recruit key customer-facing roles from a few elite universities and only hire those who "fit in".
The survey said membership to the banking industry "requires a particular portfolio of economic, social and cultural capital, which is more available to young people from privileged backgrounds".
As an example the report said "the wearing of brown shoes with a business suit is generally – though not always – considered unacceptable by and for British bankers". It also quoted one candidate, from a "non-privileged background", who was told by a mentor after an interview the tie he wearing was "too loud" to go with his suit. The candidate said: "What kind of industry is this where I can be told that I'm a good candidate, I'm sharp, but I'm not polished enough?"
The British Bankers' Association said: "The banking industry has made significant strides to improve social mobility at all levels but recognises that we cannot afford to be complacent on this crucial issue".
A number of banks have specific programmes designed to widen access, while others – including Barclays, JPMorgan and Schroders – work with social mobility bodies.
The Social Mobility Commission concluded that banks should expand entry opportunities by studying the potential of employ apprentices and other non-graduate routes and by monitoring the social background of its workforce.