Users are keen to know how fast their broadband really is
Virgin Media has criticised some broadband speed tests, saying they rely on "dirty data".
It said current tests were often inaccurate.
It is concerned that tests for 50Mbps (megabits per second) services, which are starting to launch, will be even more inaccurate.
More people are using broadband speed tests to find out whether the speed they are actually getting comes close to what service providers promise.
Most broadband consumers in the UK are currently using a service which offers speeds of up to 8Mbps but there are wide variations in the actual speeds they receive.
Virgin Media has been testing the testers and has pinpointed some issues with such services.
Online speed tests generally work by sending a file to a computer and timing how long it takes. This so-called payload is often too small, according to Virgin, to give an accurate result.
The error margin is amplified when speeds get up to 50Mbps, it said.
It is also concerned by the way web-based speed tests measure only how fast data is able to travel from one part of the internet to another, which is subject to bottlenecks and delays.
Other factors that affect results include the number of people using the test at any given time and the processing power of individual computers.
Michael Phillips, head of broadbandchoices.co.uk, said some of the issues raised by Virgin were fair.
He said he would be putting some caveats on his site's speed test.
But he believes that for the majority of users on lower broadband speeds, such tests remained an important barometer of services.
He said that the costs involved in creating an accurate test for faster speeds may be too high for those sites that make no money from the tests and simply offer them as an additional service to consumers.
"It is very costly. If you host a server you have to pay for a feed to the internet and to get one that is reliable could prove prohibitive," he said.
Virgin Media pledged to work with speed test providers to improve accuracy.
SamKnows thinks its test is more comprehensive
It recommended tests such as that devised by broadband comparison site SamKnows that uses hardware directly attached to customers' modems.
The SamKnows kit has been adopted by Ofcom and attracted thousands of triallists keen to test out the system.
It came about because the founders of SamKnows were themselves unhappy with the accuracy of other broadband speed tests.
"We wanted to make it much more comprehensive, not so much about speed as overall performance," said Sam Crawford, the founder of SamKnows.
Andrew Ferguson, head of broadband comparison site ThinkBroadband is happy his speed tester is accurate.
"We are confident that our speed tester is in a position to handle 50Mbps and faster broadband connections," he said.
At the beginning of September 2008 the site adjusted the amount of data used during the tests to ensure reliable results were provided for fast connections.
"As we test ever-faster connections, we will evolve the testing procedures," he said.
According to analyst firm Forrester only 12% of UK users have used a such a speed test.
Despite its concerns Virgin Media appears to be performing well in such tests.
Latest figures from independent broadband comparison site Point Topic put Virgin Media at the top of the league for delivering on its speed promises.