Online search giant Google has seen its second month of slow growth for "paid clicks" - the key measure of its health.

Research firm comScore says that Google's paid clicks in February rose only 3% to 515 million, compared with a year earlier.

Google makes almost all of its $16.6 billion (£8.3 billion) revenue when search users click on a sponsored-search result, with the company paying it a fee.

Google reported growth of 25% in paid clicks in its fourth quarter to December but there was no growth in January. The data will do little to calm investors' concerns about a slowdown in the firm's core business.

Those fears have already taken their toll on Google's share price, wiping $8.5 billion (£4.27 billion) in value off the holdings of each of founders Sergey Brin and Larry Page.

Lehman Brothers today cut its price target for Google shares to $580 from $644. Last night they closed up $7.41 at 458.19.

Google has upgraded its systems to try to make the ads more relevant to searches.