Google on Thursday mounted a renewed defence of the way it ranks search results, as fresh questions emerged about its practice of sometimes manually intervening to override its automated ranking system.

The company has been forced onto the defensive by the disclosure earlier this week of a preliminary anti-trust review from the European Commission. Brussels is looking into complaints from three companies that Google’s search rankings and its related advertising system treat some competitors unfairly.

In a blog posting on Thursday, Amit Singhal, a Google employee responsible for the ranking system, claimed that the company’s algorithms produced better quality and more relevant results than one based on human intervention, though he admitted the system was far from perfect.

Other Google executives have also been at pains this week to stress the impersonal nature of Google’s ranking systems in an attempt to defuse any criticism that results can be manipulated to favour or disadvantage certain sites. Julia Holtz, Google’s European corporate counsel, told journalists in Brussels on Wednesday: “We don’t whitelist or blacklist anyone.”

However, fresh evidence emerged on Thursday that Google employees do sometimes override the system, at least when it comes to the automated process that determines the weighting given to search adverts in its Adwords system.

Foundem, one of the companies that has complained to Brussels, produced two e-mails that appeared to be from Google’s Adwords employees and which discussed the issue of “whitelisting”, or altering the system to improve the ranking of a particular result.

One e-mail said: “... I am still waiting to hear back from Fred as to where we are in regards to whitelisting. I do know that he has spoken with engineering and they have been looking further at the website to build your case.” In the other, a Google employee wrote that Foundem’s adverts would do better in Google’s advertising system after the Adwords team “facilitated a change in the site quality evaluation”, overriding what is normally an automated process.

A Google spokesperson refused to comment on the Foundem claims, but acknowledged Google sometimes manually alters rankings in its search engine to counter distortions that might arise from algorithms. The spokesperson did not disclose how often such changes were made, but said they were rare.