Jacques Herzog and Pierre de Meuron will more than double the space of the world’s most visited modern art gallery by transforming a derelict part of the site, it was announced yesterday.

Their futuristic design is a chaotic-looking glass structure featuring huge blocks protruding from a pyramidal form.

It is to be constructed on the south side of the gallery, away from the river, and would be twice as high as the present roof line, at 70 metres (230ft). It would include a roof terrace with spectacular views across London and the Thames.

The Tate will seek planning approval in the autumn.

The extension, known as Tate Modern 2, has been likened by some critics to the ziggurats of ancient Babylon. Others say that it looks like the work of a child with a tub of bricks.

Its lines are slightly reminiscent of Daniel Libeskind’s Spiral extension for the Victoria and Albert Museum in Kensington. That design had to be shelved two years ago after the Heritage Lottery Fund rejected the V&A’s bid for £15 million in funding.

It has also been likened to Rachel Whiteread’s installation for the Tate Modern’s Turbine Hall of 14,000 white polythene boxes arranged in stacks.

The extension would give London a gallery with architectural ambitions to rival those of the Guggenheim in Bilbao and the Louvre in Paris.

The Tate’s director, Sir Nicholas Serota, said he hoped that it would be opened in time for the Olympics in 2012, when “the eyes of the world are on London”. He spoke of a new museum for the 21st century, a landmark building that would form one of the most exciting new cultural quarters in Europe — “a symbol of creative Britain”.

The fundraising appeal was kick-started yesterday with a grant of £7 million from the London Development Agency.

Tate Modern 2 will be funded by private and public money. The conversion of the power station involved £68 million of lottery and taxpayers’ money. The Tate will be going back for more.

Tate Modern has attracted more than 25 million visitors since it opened, making it one of Britain’s leading tourist attractions.

It was designed to accommodate 1.8 million visitors a year, but it exceeded all expectations, attracting 4 million people annually. This has led to serious overcrowding, particularly at weekends.

In 2002 it had to stay open through the night when 500,000 people crammed in to see its Picasso and Matisse exhibition.

Sir Nicholas said: “Tate needs to improve facilities so that visitors have enough space to enjoy and appreciate exhibitions and displays. Many of the comments made by visitors each month refer to the congestion within the building.”

Herzog & de Meuron has nearly 200 architects working on more than 40 projects worldwide. Among them is Beijing’s National Stadium for the 2008 Olympic Games, which is shaped like a bird’s nest.

The Tate extension would add 7,000 square metres of exhibition and display space to the present 9,000 square metres, as well as extra spaces for education programmes. It would also increase the space for retail and catering, adding six cafés, bars and restaurants to the present three.

The Tate is promoting the project as a catalyst for the further regeneration of the South Bank of the river between the London Eye and the Design Museum. At the moment, there is no public access to Tate Modern from the south, which cuts the gallery off from its surrounding community.

A southern entrance would open up a route through the building, creating a pedestrian walk from the City across the Millennium Bridge through the Turbine Hall to Southwark and Elephant and Castle. The scheme would also enable more of the Tate’s collection to be brought out of storage.

About 78 per cent of the gallery’s collection is in storage at any one time.