The campaign to build a new Young Vic theatre in The Cut received a massive boost last week when a single donor handed over a cheque for £1 million.
Patrick McKenna, a former chief executive of Andrew Lloyd Webber's Really Useful Group, made the surprise donation after a private gala performance last week of Peter Brooks's Le Costume.
Artistic director David Lan said: "We are so thrilled, delighted and grateful. It is a great surprise and a great endorsement of the Young Vic."
In February the theatre will formally announce plans to raise £12 million for the complete rebuilding of the delapidated 1970s theatre.
Young Vic supporter Jude Law could not be at the gala but the audience included Zoe Wanamaker, Joseph Fiennes, Stephen Daldry, Greg Dyke, South Bank Centre chairman Michael Lynch, Corin Redgrave and Philip Pullman.
The gala was staged to inform the Young Vic's closest supporters of its new rebuilding plans. A community open day was held on Saturday morning to enable local residents to obtain full details.
Mr McKenna left the Really Useful Group in 1997 to set up entertainment investment company Ingenious Ventures, which gives financial advice to celebrities including Victoria Beckham.