Police have arrested 27 people over scams involving the sale of fake car insurance policies.
They warned motorists to check their insurance policies carefully as thousands of drivers may have been sold cover that does not exist.
The process of selling, called 'ghost broking', often involves the use of a fake website set up purely to deceive a buyer, or adverts offering deals that are often too good to be true.
Police say those most often targeted are young people who are normally forced to pay such large amounts for genuine insurance that they jump at the chance to get what appears a good deal.
Sometimes, young people who would normally be expected to pay up to £2,000 a year for insurance, can be offered a deal for as little as £700.
They often do not find out until they have an accident, by which point they can be prosecuted for driving without insurance or have to pay a large bill for repairs.
The latest arrests were carried out by the City of London Police insurance fraud enforcement department (IFED).
The department has already had a number of earlier successful prosecutions, including one of a man who advertised insurance on website Gumtree and another who set up four fake websites offering cheap deals for which a buyer had to call.
Officers believe that despite Wednesday's arrests there are likely to be many more potential scams that have not yet been discovered.
The IFED's Detective Chief Inspector Dave Wood said: "This day of action reveals ghost broking to be a UK-wide problem which is being met by a national law enforcement response, led by IFED.
"The consequences for innocent motorists who fall victims to ghost brokers can be dire, so it is absolutely vital that drivers shopping for car insurance online, or through other means, question what they are being offered to ensure they get a real deal."
The unit has also had reports of scammers approaching people in restaurants, internet cafes and on university campuses.
The City Of London Police's advice is that if an insurance deal looks too good to be true, it probably is.
They say motorists can avoid being scammed by following advice about buying from the Association of British Insurers , or by contacting the insurer's underwriter to check a policy is genuine.