A coalition of organisations fighting the ad sharing deal struck up by Google and Yahoo way back in April will be celebrating the news that US authorities are going to launch an investigation into the plans.

The US Justice Department has announced that it is going to formally investigate the deal, which Yahoo has called an $800 million (£400m) annual revenue opportunity.

Google and Yahoo control nearly 80% of the web search market between them and so pressure groups had raised concerns about the impact on the market.

The coalition of 16 American civil rights and rural advocacy bodies had called on regulators to investigate way back in May.

Gary Flowers of the Black Leadership Forum told the BBC at the time: "We all suffer in such mega mergers".

In a letter to the Justice Department he and the rest of the coalition argued that Yahoo and Google combined would result in "a possible future in which no content could be seamlessly accessed without Google's permission".

Antitrust authorities now have 100 days to look at the deal and The Washington Post says that investigators will also demand documents from other large internet and media companies.

Both Yahoo and Google are now keeping quiet, but Yahoo CEO Jerry Yang did visit Washington last month to attempt to allay fears.