Foster & Partners, the architects responsible for the vast Bejing airport terminal that served as the international gateway to the Olympic Games, has seen a huge leap in sales and earnings on the back of a rapid international expansion.

The company, which sold a 40 per cent stake to listed UK private equity group 3i last year, filed accounts last night that showed annual turnover rose 58 per cent to £142.4m ($254m) for the year to April 2008, while earnings before interest, tax, depreciation and amortisation rose 80 per cent to £49.1m.

Lord Foster, the 72-year-old founder and now chairman, said the company had achieved record growth in the past year and that its push into international markets away from the UK had resulted in 80 per cent of its turnover coming from overseas projects.

In spite of this, Mouzhan Majidi, a long-time Foster partner who was elevated to chief executive after the reorganisation last year, said that the company had made a net loss of £15m after interest, depreciation and a goodwill amortisation charge of £15.5m that results from the 3i investment and will run for the next 20 years.

As well as its international expansion, the firm has broadened out beyond architecture into other areas such as design and management consultancy.

“We are now working across five continents completing projects as diverse as a writing pen by the product design team for Diplomat, a new Elephant House for Copenhagen Zoo and the world’s largest and most advanced airport building, the £1.8bn Beijing International Airport Terminal 3, which opened successfully in February,” Mr Majidi said.

Mr Majidi added that the marriage with 3i had been good and that there had not yet been any discussion as to the timing or manner of any potential exit strategy for the private equity group, which would typically look to a three to five year time-frame.

Steven Nicholls, 3i Partner, called the results “impressive” and said he was “proud to be part of this continued success story”.