Google’s latest site update – named Panda – is reported to be having a significant impact on the visibility of certain websites in natural search results.

Changes to the algorithm that Google uses to decide how websites are ranked in natural search results were made in the US in late February. At the start of this week, the Panda update was extended to all of Google’s English-speaking sites, including Google.co.uk.

Google says that the update is designed to help users find “more high-quality sites” in search. The changes appear to be having the most notable impact on sites that are content-heavy. Those who re-use content that’s available in other places are said to be suffering in search results.

Google admits that it is also beginning to incorporate information about the sites that its users block from search results into its algorithm. It says that the upshot is that it is becoming more accurate at detecting site quality. But, as always, the search engine gives away relatively little about how the algorithm works.

Analysis by Searchmetrics has determined content-heavy sites that are winners or losers from the update. Among the 100 biggest losers are Play.com and Mydeco.com. Many of the shopping comparison sites – such as ciao.co.uk and twenga.co.uk – as well as a number of voucher code sites have also come off badly.

In comparison, Ebay, Amazon, and Argos’ UK sites were among the 100 biggest winners (as was Retail-week.com).

If your site is heavily dependent on either traffic from such content-heavy sites, or directly from Google natural search results, it’s worth keeping an eye on how the Panda changes affect performance in the next few weeks.