Private equity firm 3i has acquired Foster + Partners for about £350m. The announcement comes after months of speculation over the sale of the 850-strong firm, which celebrates its 40th anniversary this year.

Lord Foster, the architect behind London’s Swiss Re ‘Gherkin’ and the Hearst Headquarters in New York, has appointed his protégé Mouzhan Majidi, who has led projects that include the new Beijing Airport, will become chief executive in the new structure. The news comes after months of speculation over the sale of a stake in the 850-strong practice.

Lord Foster had owned more than 90% of the firm’s equity. It is understood that senior staff are expected to be offered an option to buy shares once the deal has gone through. Earlier in 2007 Foster unwound an employee share trust and bought back his employees’ 49.6% stake in the firm.

“How do you bring to bear the foundations for something that builds on that for the next 40 years and beyond?” Lord Foster said yesterday in an interview.

Foster decided to bring in outside investment and broaden the shareholder base in order to realign the practice. The firm is looking to expand internationally and in areas such as consultancy, project management and product design.