Greg Smith, the former Goldman Sachs employee who infamously quit Wall Street via a New York Times article in March, says the investment bank routinely took advantage of charities and pension funds in order to increase its profits.

In an interview on CBS News' 60 Minutes on Sunday to promote the release of his book called Why I Left Goldman Sachs: A Wall Street Story, Smith said that securing an unsophisticated, or "muppet" client was the top goal of the bank's salespeople. His frustrations with that culture meant he "literally wanted to hit the board of directors over the head".

"Getting an unsophisticated client was the golden prize," he told the programme. "The quickest way to make money on Wall Street is to take the most sophisticated product and try to sell it to the least sophisticated client.

"What Wall Street will do is they will approach one of these philanthropies or endowments or teachers' retirement pension funds in Alabama or Virginia or Oregon and they'll say to them: 'We have this great product that is going to serve your needs'. And it looks very alluring to these investors but what they don't realise is that upfront they are immediately paying the bank $2m (£1.2m) or $3m because of their lack of sophistication."

Smith was no more than a mid-ranking employee when he penned his March article, which is best remembered for his claim that Goldman bankers in London frequently dubbed unsophisticated clients as "muppets".

"Within week one [of arriving in the London office] I met a junior guy who was 24, 25 years old and the first thing he'd told me was that he had just traded a sophisticated derivative with a 'muppet client' who'd paid the firm an extra million dollars because the client was so trusting that he didn't check the price with other banks," Smith recalled. "Now you could think to yourself, is this some rogue guy who is just talking callously about clients, but his boss who's a managing director was sitting right next to him nodding and chuckling along."

Smith also recalled how he and a Goldman partner travelled to Asia to meet a major client, described as the "head of one of the biggest funds in the world".

"[The client] looks me and the partner in the eye and says: 'Let me be honest with you guys. We don't trust you at all. But don't worry, there's nothing to worry about, we're going to keep doing business with you because you're the biggest bank, you're the smartest and factually we have to do business with you'.

"Now my jaw almost dropped because hearing from one of your biggest clients that they don't trust you when your whole mantra and reputation is built on trust, to me was the worst possible thing that you can hear. And then I leave the meeting and the partner from Goldman Sachs, who I was with, is jubilant. This is great news. The client is going to keep doing business with us because they have to."

Goldman declined to give CBS a response to its Smith interview. However, the bank's board has already been told that an internal investigation triggered by Smith's resignation – which was dubbed the "muppet hunt" – had found little substance to the allegations.

The bank has also been briefing that Smith had been angling for a promotion and for the bank to double his pay to $1m, although Smith claims he would have quit even if his pitch had been successful.

"What I can say to you, and this may sound stupid, is that I didn't go to Wall Street purely to make lots of money," he said. "I definitely wanted to make money, but I left because things had veered so far from what I actually believed was right. I could have just left and walked out and said nothing about it but I would have felt that that was not the right thing to do."

Along with several other major Wall Street banks, Goldman has been widely criticised for making money at the expense of clients who did not fully understand the complex financial products they were dealing with.

In 2010 Goldman was fined $550m as part of a settlement with the Securities and Exchange Commission – the largest in the US regulator's history – when the bank accepted that the marketing materials it issued to investors for its infamous Abacus transaction gave "incomplete information".

The Abacus case had called into question the integrity of Wall Street after the commission alleged Goldman had packaged up mortgages into Abacus and then sold them to investors without telling them that one of its powerful clients, the hedge fund Paulson, had been taking a trading position intended to profit from a fall in the value of US house prices and had even selected some of the mortgages included in the product.