Google is facing a wide-ranging investigation into its online advertising system by the US justice department, which could cost the technology giant at least $500m (£304m).

The justice department has launched an investigation into whether Google's lucrative advertising system unfairly treats some unnamed advertisers.

Google is facing a similar probe by the European commission, as its multibillion dollar business model comes under increased regulatory scrutiny around the world.

The potential $500m cost of the US probe came to light in Google's quarterly report to the US Securities and Exchange Commission on Tuesday.

Google said in its filing: "In May 2011, in connection with a potential resolution of an investigation by the United States department of justice into the use of Google advertising by certain advertisers, we accrued $500m for the three month period ended March 31, 2011.

"Although we cannot predict the ultimate outcome of this matter, we believe it will not have a material adverse effect on our business, consolidated financial position, results of operations, or cash flows."

Google's advertising network, which primarily delivers short text ads alongside relevant searches, accounted for $8.3bn in sales in the first quarter of this year.

The company reported an overall 18% rise in profit in the first three months of this year, to $2.3bn. However, the $500m set aside earlier this month reduced Google's net income to $1.8bn, the company said in its SEC filing.

European regulators have opened an investigation into whether Google unfairly manipulates its search results to boost its own products, and whether its advertising network is rigged to drive up prices.