Google's advertising system is under investigation by the US Justice Department and it has put aside $500m to settle any potential charges, the Internet search engine revealed on its website on Wednesday.

The charge decreased the net income to $1.8bn, or $5.51 per share for Q1, Google said in a filing with the SEC late on Tuesday. It had earlier reported net income of $2.3bn, or $7.04 a share for Q1.

Google said: “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.”

The company said it cannot predict the ultimate outcome of this matter but believes it will not have a material adverse effect on its business, consolidated financial position, results of operations, or cash flows.