People could be left significantly out of pocket if they follow their bank or building society's advice about ensuring their savings are protected by the Government's official compensation scheme.
The danger was highlighted by research from a consumer group which sent undercover staff into branches of the 13 largest finance providers in the country.
The investigators posed as new customers with £100,000 to deposit into a savings account. Shockingly not a single member of bank or building society staff told them about the Financial Services Compensation Scheme, or warned that only £85,000 of the £100,000 total would be protected in the event of a bank's collapse.
It means that anyone heeding the advice would leave part of their savings at risk of disappearing if there was another banking collapse.
Richard Lloyd, of Which? – the group that conducted the investigation – said: "It is inexcusable that bank staff can't give customers basic information about the compensation scheme if their bank goes bust.
"In the event of a collapse, this bad advice could cost people many thousands of pounds from their life savings. We hope this is a wake-up call to the banks that they need to improve staff training."
Even when prompted, most banks and building societies could not say what the correct amount protected by the FSCS is. Several admitted they had no idea and a third of HSBC workers got it wrong.
Meanwhile many staff didn't explain that if you have savings with more than one member of the same banking group, the full amount may not be covered. Only one provider, First Direct, managed to score more than half marks on the question.
Overall HSBC came bottom with a score of just 51 per cent of staff knowing about the FSCS while Yorkshire Building Society came top with 88 per cent, followed by Britannia Building Society (82 per cent) and Halifax (81 per cent).
The British Bankers Association said the results were disappointing. A spokesman said: "All employees are made aware of the FSCS and many banks have relevant training, so it is disappointing that more staff did not communicate the benefits of the scheme when asked."
It promised to make sure that customers are given accurate information on FSCS.