ARM's incoming chief executive has vowed to resist any takeover attempts and keep the fierce independence that has helped transform the Cambridge startup into the world's largest microchip designer.

Simon Segars, president of ARM, will take the top job in July when Warren East steps down after 12 years, the company said on Tuesday in a surprise announcement.

Segars, who joined ARM in 1991 when he was 23, said: "We have 300 licensees of our technology, we share confidential information with each other and they rely on the neutrality of our position. Being an independent company is the right model."

East, 51, has been one of Britain's most successful listed company bosses, having overseen the creation of a $20bn (£13bn) business. A low-key executive who plays the organ in his village church, he has described himself as a "luddite when it comes to gadgets". East propelled ARM to international success by putting it at the heart of the smartphone boom.

Discretion and neutrality were key to his strategy. ARM creates designs that can be licensed to any other company, but has never ventured into manufacturing chips itself, unlike its arch-rival Intel. The result is that ARM's designs are in 32% of semiconductors sold worldwide, double Intel's 16%.

East has never spoken much about his most important client, but ARM's business was transformed by the use of its chips in Apple products, particularly the iPhone.

Created as a spinoff from Acorn Computers, in which Apple took a 30% stake for $3m in 1990, Advanced Risc Machine, as it was known, joined the stock exchange in 1998, after which Apple gradually exited its holding while maintaining the collaboration.