The president of Sony Ericsson, the world’s fourth-biggest mobile handset maker, quit unexpectedly yesterday, raising concerns about the joint venture’s future.

Miles Flint, who joined Sony Ericsson from Sony Europe three years ago, is to leave for personal reasons.

Sony Ericsson said that Hideki “Dick” Komiyama, chairman of Sony Electronics USA, would take over Mr Flint’s role on November 1. He will also be based in London.

Richard Windsor, an analyst at Nomura, described Mr Flint as the “visionary” behind the group and said that the development was “negative” and raised some “uncertainty”.

The venture, he said, was at a critical phase of its development, as it prepares to branch out into smartphones and to make a further push into the cheaper end of the market.

Success in the low-end, prepay arena is deemed critical if the group is to extend its market share beyond its current 10 per cent level.

Ben Wood, of CCS Insight, the telecoms research group, said that Mr Flint’s contribution to the success of the venture “should not be underestimated”.

Sony had tried and failed in the mobile arena several times before it joined forces with Ericsson in 2001.

Under Mr Flint, the venture thrived and moved into fourth place in the mobile market, behind Nokia, Samsung and Motorola.

Among his key achievements was the revival of the Walkman brand to create music-friendly mobile phones.

Mr Flint is to remain as an executive adviser to his successor until the end of December.

In a statement, he said: “After nearly 17 years in Sony and Sony Ericsson, I wish to develop new personal and business opportunities and intend to take a short career break before deciding on my next steps.”

In the second quarter, Sony Ericsson reported a 55 per cent jump in quarterly profit to €327 million (£221 million), although that was below market expectations. Its average selling price fell to €125 from €145 a year earlier.

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