Many broadband speed testers can’t give accurate results when used to test high-speed broadband services, said Virgin Media.

The cable giant said its research had found lab tests of its 50 Mbits/sec service have shown serious discrepancies between the results and the actual speeds users were likely to be getting.

It said even users of current 20 Mbits/sec services can be given readings that don't reflect the user's true connection speeds.

This could have serious consequences for ISPs who increasingly rely on the speed they can offer in this hugely competitive market.

Jon James, director of group strategy for Virgin Media, said: "Customers increasingly rely on speed test comparisons to choose their broadband provider. "

According to Virgin the problem is the way these speed testers handle data. To get a result they test how quickly a small file is uploaded and downloaded to the user's PC. However, Virgin contests this can’t give a reliable result.

Michael Phillips, Product Director of ConsumerChoices.co.uk said:"Speed testing is usually an accurate and useful tool for consumers to check that they are getting what they are paying for. However, the new super fast 50Mb product Virgin Media is launching is ahead of current testing technology.

"Consumers come to us for reliable information and we share Virgin Media's desire to improve the accuracy of the 50Mb speed testing. We will work with Virgin Media to explore ways that we can develop our testing methodology for high speed services as they near the launch of their
upcoming 50Mb product."

The problem Virgin contends is the file size used is too small and data often needs to travel across many parts of the internet to get to the end user, causing delays. It also said that like any site on the internet, web based speed testers are subject to congestion; if too many people are testing speeds at the same time the result will be skewed as the speed test site isn't able to send data fast enough.

It said only a hardware speed test solution that attaches to the customer’s modem, such as the hardware adopted by Ofcom. This has been developed by Sam Knows (a broadband comparison site), and is able to test the speed of a connection directly.

The ISP said while it was not criticising the sites hosting the speed testers, when very high-speed broadband, such as its 50 Mbits/sec service, is adopted widely customers will demand accurate readings.

“We are working with speed test sites to help them accurately measure all broadband services, including our new superfast offerings. We will also be helping customers to understand the accuracy of the speed test information available to them so they can make well-informed choices,” said Mr James.