Search engine giant Google has agreed to buy web advertising network Doubleclick in a $3.1bn (£1.6bn) deal.

The cash buyout, the biggest in Google's history, will allow the company to tighten its grip on the advertising market.

Doubleclick helps link up advertising agencies, marketers and web site publishers hoping to put ads online.

In recent years, Google has been expanding into print, radio, video, mobile and TV ad markets.

News of the deal comes just six months after Google paid $1.65bn for video sharing website YouTube.