Herzog and de Meuron Architekten's extension for London's Tate Modern gallery has received a £50m pledge from the UK Government.
But the government's pledge will only be met if the Tate Modern can raise the rest of the funds needed to build it, despite it already being given planning approval.
In total, the project is estimated to cost £215m.
UK Culture Secretary James Purnell has praised the design of the Pritzker Prize-winning firm.
"Herzog and de Meuron's astonishing design will create a stunning new landmark for the capital, and a brilliant space for showing the very best of international modern art to the world, free of charge," Purnell says.
The extension will increase the gallery’s premises by 60 percent.
Construction is planned to commence in 2009 and is due for completion in time for the 2012 Olympics.
By Penny Jones