'If it's not affecting your work in an obvious way and if you are bringing in a lot of money it is likely to be overlooked,' says the high-ranking insider. 'For most banks the question is simply one of deciding which drugs to tolerate.'
This admission comes despite the fact that ever more institutions in the City of London are introducing testing for employees, supposedly to curb drug abuse. City Medical, a company specialising in employee drug testing, says it has seen a 20% increase in demand for its services in the past year.
Meanwhile, the City's drug of choice - cocaine - shows no sign of losing popularity. A UBS Warburg employee recently died of a cocaine overdose and anecdotal evidence suggests that the heady mix of youth, ego, money and pressure is making London's financial centre a fertile ground for drug abuse.
Leading the field in testing are US banks, which frequently require interview candidates to submit a urine sample for testing prior to making an employment offer.
In the US this is standard practice with most large employers - more than 80% of Fortune 500 companies use drug testing for employees, according to a 1998 poll.
However, testing positive for drugs does not necessarily mean you will not get a job in London's financial district. According to Wendy Poulter, vice-president of human resources at Chase Manhattan in London, Chase would not employ someone who had just failed a test, but might consider them later if they produced a drug-free result.
Such policies arguably make a mockery of the whole testing process, particularly since drugs only stay in the system for a relatively short period.
According to Dr Gareth Spiers, a doctor at City Medical, cannabis is the drug that hangs around the longest, and typically can be detected three weeks after use. By contrast, he says: 'If you took cocaine or ecstasy on Saturday, then on Tuesday you will test clear.'
Since most City firms using drug tests give prior warning of the tests, it is not difficult to evade detection. 'You'd have to be really stupid to fail a medical,' says one insider.
On top of that, there is a wealth of information on the internet about how to get round drugs tests, using tablets of concentrated, dried urine or herbal detoxifiers.
Spiers at City Medical claims such products are useless - and also dismisses scare stories about the inaccuracy of drug tests. These range from suggestions that taking the pain killer Ibuprofen can make you test positive for marijuana, to a reported case of someone testing positive for heroine after eating a poppy seed bun for breakfast.
Factors like these, says Spiers, can easily be identified by interviewing candidates who produce an ambiguous result.
Nevertheless, the issue of testing is set to become even more contentious from October this year, when the new Human Rights Law comes into force. According to Diane Luping, a legal officer with civil rights group Liberty, employees will be able to argue that drug testing infringes their privacy.
An employee's case will be especially strong, she says, where it cannot be shown that possible drug use has in fact affected their work performance or put at risk the health or safety of others.