The 71-year-old starchitect said the long-awaited deal - which values the company at £350 million - would secure the future of the firm and enable him to 'take a step back from the day-to-day running of the company' and allow him to 'concentrate on design research and strategy'.
Foster - who had held a 90 per cent stake in the firm - will head the new board as chairman; senior partner Mouzhan Majidi becomes chief executive. Original shareholders Spencer de Grey, David Nelson and Graham Philips all become senior executives.
Speaking of the 3i deal, which will help fund Foster & Partners' expansion into China, India and Middle East, Foster said: 'Traditionally people only end up in this kind of situation to be bailed out, and here we are going down the runway and it is the equivalent of putting on the afterburners - as if we are turbo-charging ourselves.
'We are doing this when we are on a creative high and on a very solid commercial base in terms of our order books. It is not something that we have to do - it is not provoked by some terrible crisis,' he added.
When asked if the firm's name would remain as Foster & Partners for at least the next 40 years, the man behind iconic buildings such as London's Gherkin and Millennium Bridge, France's Millau Viaduct and Hong Kong's Chep Lap Kok airport, said: 'Absolutely, I'm looking forward to my children joining. So yes. And in that sense it is a vote of confidence on my part in the team now, and the team in the future.'