"After an analysis of the market test that was concluded on June 27, I concluded that the proposals that Google sent to us are not enough to overcome our concerns," he said.

"In this sense, I wrote a letter to Google, to Mr Schmidt (Google's executive chairman), asking Google to present better proposals, to improve its proposal," he added.

But he did not reveal what the deadline would be for Google's next offer.

Google spokesman Al Verney said the company would work with the EU, though he insisted that Google's offer in April "clearly addresses the four areas."

The EU's competition concerns focus on how Google favours its own services in its search results and displays content from other websites.

It also wants Google to change how it manages adverts appearing next to search results and make it easier for companies to buy adverts through rival networks.

The EU executive has conducted a three-year investigation into claims that Google used its 95 percent market share in Europe to distort Internet search results by putting links to its own products and services at the top.

For his part, David Wood, the Legal Counsel for ICOMP, a software industry lobby group whose supporters include Google rival Microsoft, said that it was “reassuring that the commission has recognised, as had been argued by many, that Google’s offer of proposed remedies was inadequate."

"By providing Google with the opportunity to go back to the drawing board, the commissioner has offered a unique opportunity to meet the concerns expressed not just by the commission but also by many third parties," he added.

Under EU competition rules, Google could face a potential fine of up to 10 percent of its €40 billion annual turnover, comfortably higher than the largest existing EU anti-trust fine, the €1.6 billion fine levied on rival software giant Microsoft.