The research revealed that two thirds of the bank staff from HSBC, RBS, Lloyds Banking Group, Barclays and Santander who had a sales role and sales targets said there is now more pressure than ever to meet targets.
Which? chief executive Peter Vicary-Smith said: "This proves the need for big change across the industry and for bankers to put customers first not sales. We're calling on the banks to be much more transparent about their sales targets and incentives."
The dominant sales culture in Britain's banks is encouraging mis-selling, says Which?. Almost half of those surveyed, 46%, said they knew colleagues who had mis-sold products in order to meet targets. And four in 10 felt they were sometimes expected to sell even when it was not appropriate for the customer.
Some high street banks said they are making moves to reduce sales targets which was reinforced by 41% of bank staff who said they had noticed a decrease in the availability of incentives.
However 81% said that the pressure to meet sales targets had stayed the same or increased as two thirds of staff in sales roles said they were sometimes or always told to sell more.
Vicary-Smith said: "Our survey reveals the stark realities of the sales culture that still exists at the heart of the banking industry. Senior bankers said the culture is changing but this shows it just isn't filtering through to staff on the front line who remain under real pressure to put sales before service even after incentives are taken away."
Which? is calling on all the banks to refocus their incentive schemes on customer service as Barclays and The Cooperative bank have done.
Further research from Which? found that customers were feeling the effects of the pervasive sales culture in Britain's high-street banks. Four in 10 said the last time they contacted their bank they were offered a new product or service that wasn't suitable and a quarter; 25%, felt pressurised to take it.
Craig Bernhardt, chief executive of claims management company EMCAS, said: “Despite the huge amounts that these banks are currently re-paying to consumers for mis-sold payment protection insurance it’s disappointing to see that they are still continuing to encourage a culture that breeds mis-selling. It’s not fair on consumers and it needs to stop."
Which? wants to see big changes in the banking industry to help restore consumers' trust as only one in 10 people said they trusted bankers to act in their best interests.
Which? is handing in a dossier of evidence to the Parliamentary Commission on Banking Standards, government, opposition ministers and the Financial Standards Authority.
The dossier includes the bank staff survey, consumer views and previous research on the banking industry. This is in addition to 120,000 signatures from the general public pledging their support for the "Big Change" campaign in banking with Which? and 38 Degrees.