Analysis by Which? found that more than 96% of crimes reported to Action Fraud are closed without a successful outcome.

Action Fraud, a branch of the City of London Police, is a centralised reporting system for victims of fraud.

Ceri Stanaway, Which? Money Editor, said: “The shockingly low success rate for fraud investigations is leaving many victims deprived of justice and suggests the authorities are fighting a losing battle against this type of crime.

“Unfortunately, investigations are often conducted at a slow pace – with communication between banks and the authorities often dragging on for weeks before police can launch a full inquiry.

“We’d urge consumers to be extremely cautious when dealing with unsolicited contact – as fraud is on the rise.”

Freedom of information requests by Which? found most UK police forces have seen a substantial drop in their success rate for investigating fraud – with some solving more than 40% fewer cases in 2016 than in 2014.

This comes at a time when online fraud is on the rise. Fraud and computer hacking are now 10 times more common than burglary with overall fraud estimated to be approaching £200bn a year, according to Action Fraud.

Which? said the police might struggle because of budget cuts and fraud being a difficult crime to investigate.

The consumer group also found a lack of transparency around fraud statistics. While success rates for most types of crime were published in the Home Office Crime Outcomes report, the figures for fraud were buried in a submission by City of London Police to the Home Affairs Select Committee.

City of London Police said: “Fraud is the fastest growing crime in the UK and this means that it is not always possible to enforce our way out of the problem; due to the high volume of cases it is not possible for every report of fraud to result in a judicial outcome. For this reason, it is important that law enforcement not only focusses on pursuing suspects, but also works to prevent and protect people from fraud.”

It added: “The City of London Police’s National Fraud Intelligence Bureau disrupts email addresses, bank accounts and phone lines associated in fraudulent activity. In 2016/17, 170,856 disruption requests were made.

“Action Fraud and the National Fraud Intelligence Bureau receive and assess reports of fraud, which are then sent to law enforcement for investigation. This process can take some time as we may sometimes need to submit a Data Protection request to a bank, the banks’ timeframes to respond can vary and sometimes take weeks to complete. Some reports may also be linked to each other and therefore it takes a long time to assess the intelligence. Action Fraud also receives a high volume of reports (approximately 40,000 reports per month).”

Action Fraud is currently preparing to introduce an upgraded computer system which will improve the service for victims.