London's Tate Modern gallery has received a £5m donation towards its extension plans from banker and philanthropist John Studzinski.
The new building is expected to cost about £215m
Tate executives want to add an 11-floor glass building to the south side of the existing gallery at Bankside.
Mr Studzinski said he is "delighted" with the concept of the development.
Tate director Sir Nicholas Serota said the donation was a "magnificent act of faith in a project that will transform the building".
He added: "John Studzinski was a generous benefactor of Tate Modern in 2000 and has now made a gift which will be the cornerstone of the next phase of the development."
It is hoped the development will be ready to open by 2012
It is hoped the site will provide more space for the Tate's collection and will help combat overcrowding at the gallery.
The development, which is expected to cost about £215m, has been given £7m by the London Development Agency.
Meanwhile school children and teenagers are being asked for their ideas on how to improve the extension plans.
Artistic workshops are to take place at the gallery on 29 May.
About 150 young people will be encouraged to explore different options for the new development.
The event will culminate with the first ever "giant sleepover" in the Turbine Hall in tents which the children will be asked to paint on.