Some UK broadband customers are getting just 48 percent of the download speeds advertised by their ISP, according to broadband comparison site Broadband Expert.

Broadband Expert tested 41,230 connections during February and March, and compared the actual speed customers were achieving with the maximum speed quoted for their broadband package. It identified a significant difference between the speeds providers were quoting and those achieved by users.

In the 16Mbits/s+ category, the average actual speed achieved was 8.6Mbits/s, just 45 percent of the quoted maximum speeds. However, customers on 2Mb packages achieved on average 1.8Mbits/s, or 88 percent of their quoted maximum speed, showing that those paying for packages with lower maximum speeds were much more likely to get close to the advertised speed.

Broadband Expert technical director, William Harvey commented: "It seems clear that providers are promising speeds far greater than they are capable of delivering, especially with top-end packages. If ISPs set a realistic expectation of the speeds that can be achieved, they are much less likely to upset consumers than those who advertise fast speeds and under deliver."

Harvey said contention ratio, or the maximum number of people that can share a connection, is one reason for the difference between quoted and actual speeds. One of the top performers in the tests - UK Online - has contention ratios of 33:1 and 20:1 (dependent on the package chosen) compared to the industry standard of 50:1.

"Broadband providers operating on higher contention ratios are likely to see their connections slow down at peak times when more people are online," added Harvey.