Nokia, the world's leading maker of mobile phones, stepped up its battle with Microsoft yesterday by taking control of Symbian, the leading supplier of smart-phone operating systems.

The Finnish group, which was already Symbian's biggest shareholder and customer, plans to buy 31.1 per cent of Symbian from Psion for an estimated £135.7 million (€198.86 million) to take its total stake to 63.3 per cent.

The move will give Nokia more influence over Symbian's development as it battles to stop Microsoft gaining the same dominance in mobile software as it has in personal computers. Symbian has 95 per cent of the fast-growing smart-phone market.

But analysts said there could be a backlash if Symbian was seen as Nokia's proprietary system, rather than an open standard with broad support in the handset manufacturing community. Symbian's other shareholders include Sony Ericsson, Ericsson, Samsung, Siemens and Panasonic.

Psion shares fell 30.75p to 65p, as investors reacted to the sale of the company's most important asset and the price achieved.

Nokia is paying an initial £93.5m for Psion's Symbian stake and will pay a further 84p for every device sold with a Symbian operating system in 2004 and 2005.

Analysts expect around 18.7 million Symbian devices to be sold in 2004 and 31.5 million in 2005. The agreement values Symbian at £436.2 million, more than a third higher than the £300 million it was valued at last autumn when Nokia and Psion agreed to buy Motorola's 19 per cent Symbian stake.

Mr Antti Vasara, Nokia vice-president of technology sales, said the company wanted to ensure that Symbian had "staying power in an extremely competitive market". He admitted that Nokia and Symbian could be regarded as the same entity, potentially deterring some users.