The world's top cellphone maker Nokia cut its forecast for second half profitability and 2009 market share at its key phone unit on Thursday, sending its shares sharply lower.

Nokia, which is facing tough competition from the likes of Apple , Samsung <005930.KS> and RIM , scaled back its second-half underlying operating profit margin forecast for its key phone unit to the first-half level of 11.3 percent, from 13-19 percent previously.

Nokia also cut its forecast for 2009 market share at its phone business, seeing it now on a par with last year, compared with an earlier forecast for a rise.

Shares in Nokia fell more than 8 percent on the news to 10.18 euros, compared with a 2.8 percent lower DJ Stoxx technology index <.SX8P>.

(The reaction) is the impression that Nokia concedes that the competition in the market place is heating up in the second half ... that is related both to the reduced margin outlook and the market share outlook, said West LB analyst Thomas Langer.

Nokia's underlying earnings per share slumped to 0.15 euros from 0.37 euros, but beat the average forecast of 0.13 euros in a Reuters poll of 31 analysts.

The handset industry this year is facing its worst downturn ever, and market No 5 Sony Ericsson <6758.T> reported earlier on Thursday a deep loss for April-June.

Amid the doom and gloom Nokia have delivered some excellent results ... Nokia's high-tier performance continues to be the biggest concern, said CCS Insight analyst Geoff Blaber.

Nokia also said its telecom equipment arm, Nokia Siemens Networks , had won a 1.1 billion euro ($1.55 billion) order to operate the telecoms networks of Brazilian operator Oi over the next five years.

(Reporting by Tarmo Virki, editing by Elaine Hardcastle)

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