Google failed to grow its share of the US search market for the first time in 11 months in July, as the number of internet searches by Americans jumped 30 per cent from a year earlier.

New research from comScore Networks shows that Google's share of the US search market, the world's most lucrative, fell to 43.7 per cent last month from 44.7 per cent in June. Against the same month last year it was up by 7 percentage points.

Americans conducted 6.3 billion searches online in July, up 30 per cent from the same month last year.

In second place, Yahoo! increased its market share marginally to 28.8 per cent in July from 28.5 per cent in the previous month.

Microsoft held third position with 12.8 per cent share of searches. Time-Warner, which owns AOL, took 5.9 per cent of the market and Ask Network 5.4 per cent.

Robert Peck, the Bear Stearns analyst said: "We think [the] loss in domestic market share could signal a topping point for Google. We do not expect Google's aggressive market share gains to continue in perpetuity."

Shares in Google were down 1.5 per cent at $377.7 in pre-market trade today. The shares have fallen from more than $420 early last month and from more than $470 in January.

Wall Street has reacted nervously this year to suggestions that Google's prodigious growth faces checks.

In February, comments from finance chief George Reyes that "at the end of the day, growth will slow" helped wipe off $8 billion (£4.5bn) off the company's market value in minutes.

Google, which had faced criticism for not giving forward-looking statements, was forced to give a clarify Mr Reyes' remarks.

It said: "our revenue growth rate has generally declined over time and we expect that it will continue to do so as a result of the difficulty of maintaining growth rates on a percentage basis as our revenues increase to higher levels".

Ed Stenson and Jon Lewis at Schaeffer’s Research wrote this spring: "Unless investors imagine that extraterrestrials are going to be using the search engine soon, the kind of growth rates we’ve seen recently have got to slow down."

Companies such as Google make money from online searches by delivering targeted adverts to consumers when they search for certain terms.

The comScore tracks the internet habits of 2 million volunteers across the United States.

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