Scam victims are being left on hold for more than an hour before they can speak to someone at their bank to freeze their account, putting life savings at risk.
High street banks have blamed staff shortages, the pandemic and more complex scams for the long wait times, but experts have warned they are failing fraud victims.
Customers who realise they have fallen victim to a scam are advised to contact their bank immediately, as quick action can sometimes mean money can be recovered or accounts frozen before funds are stolen.
But distressed victims are being met with mammoth waiting times before they can take vital action to protect their money.
Customers must wait an average of more than 10 minutes to speak with the fraud team at five high street banks, including Lloyds, Santander, TSB and Barclays, according to research by consumer website Which?. The Co-operative Bank was the worst performer, with customers waiting an average of more than 31 minutes.
Which? tested the fraud helplines of 11 banks and building societies throughout different times of the day over the course of a week in March.
On two occasions it took more than an hour for a call to the fraud team at the Co-operative Bank and Santander to be answered. The maximum wait time at Lloyds, TSB and NatWest was more than half an hour.
Meanwhile customers of First Direct only had to wait an average of 16 seconds before their call was answered and less than two minutes at Nationwide.
The Co-operative Bank blamed recruitment challenges and increased levels of sickness and employees isolating at home for its increased call wait times. It said: "We have reduced call wait times across our telephone services for customers since the research was carried out by Which? and we now have average call times of under three minutes for customers contacting us to report fraud."
Rocio Concha of Which? said: "Businesses blaming poor customer service on the pandemic has been frustrating enough for consumers, but it is unacceptable to do so when distressed victims call fraud helplines to report a crime and are made to wait long periods – and in some cases are paying to do so."
Customers left on hold are also racking up charges in the meantime. Six of the 11 banks included in the investigation charged fraud victims a fee for using their helpline that could lead to "hefty phone bills", Which? warned.
A Santander spokesman said the bank had seen a "clear shift in the complexity of scams" and it was spending longer speaking with customers. It added: "To combat this, we are actively recruiting more colleagues and by the end of the year, will have an additional 400 colleagues in place to support customers."
Meanwhile a Lloyds Bank spokesman said calls to its fraud line in March were answered within seven minutes. "Gathering information on fraud can be complex and, while we never want anyone to have a long wait, it’s important we spend the right amount of time on every call," the bank said.
A spokesman for TSB said the average call waiting time on its fraud line was under six minutes in March, whilst the majority of customers benefit from the company's "fraud refund guarantee" policy. NatWest failed to respond in time for publication.