The campaign to build a £215m extension to Tate Modern received a big funding boost yesterday when the government announced a £50m grant to kick-start the project.

James Purnell, culture secretary, said the money would help create 'a brilliant space for showing the very best in international modern art to the world, free of charge'.

He added that the new building would 'prove once and for all that London is the cultural capital of the world'.

The extension by the architects Herzog & de Meuron – a stack of rectangular boxes shaped into a glass pyramid – would be a 'stunning new landmark', he said.

Tate Modern is already the most popular museum of modern art in the world, attracting 5.2m visitors last year and the UK’s most popular tourist attraction after Blackpool Pleasure Beach.