The Gherkin is lauded the world over. So why is it still empty?

It is one of the few modern buildings to instantly capture the hearts of the public. Norman Foster's glass "gherkin", with its graceful, tapering steel curves, dominates London's skyline and has been hailed as a triumph of design and engineering.

But six months after it opened, the cigar-shaped tower, visible from almost any part of the capital and one of the leading contenders for the most prestigious accolade in architecture, is still half empty, to the consternation of the owners and developers.

Bookmakers had installed the Swiss Re building, officially known by its address of 30 St Mary Axe, as the clear favourite for the Stirling Prize, the winner of which was due to be announced last night.

But while the "erotic Gherkin" is widely loved from the outside, the inside has problems: only the first 19 floors of the 40 floors are occupied and those are by employees of Swiss Re itself, the reinsurance company that commissioned the £130m skyscraper. The building's association with the company is believed to have deterred other tenants who feel their own brand would be overwhelmed by it.

The tower faced competition in a vintage year for breathtaking, sometimes controversial, modern architecture. Second favourite was the stainless steel Spire in Dublin, a 120m needle. Designed by Ian Ritchie, it replaced a monument to Nelson destroyed by the IRA in 1966, and is the first structure in Ireland to be short-listed for the prize.

The bulbous blue Kunsthaus arts centre in Graz, Austria, designed by Peter Cook, a former member of the 1960s radical pop-architecture group Archigram, was another contender. Bulging through thered-tiled rooftops of the historic city, it is known as "the friendly alien".

Lord Foster's Business Academy, in Bexley, Kent, one of the Government's new specialistschools, was also on the short-list judged by a five-strong panel including the sculptor Antony Gormley. A classic modern glass box, it is so popular with students that attendance has soared.

The Imperial War Museum North in Manchester is in the distinctive style of the internationally renowned architect Daniel Libeskind, the master planner of the Ground Zero rebuilding in New York. Its sharp angles and curved blocks represent "the contemporary world shattered into fragments and reassembled", the architect says.

The final competitor was the Phoenix Initiative in Coventry, by MacCormac Jamieson Prichard, a £10m project to create and link public spaces in the city, regenerating the cathedral area and redefining its skyline.

But winning architecture's premier prize is unlikely to bring tenants flooding into 30 St Mary Axe, despite the blaze of global publicity it will receive. Among its many innovations, the tower, which can accommodate 4,000 people on its 40 storeys, has a double skin of steel and glass that traps heat and sunlight, making it 50 per cent more energy efficient than its rivals. But the circulation of air makes it difficult to split up floors and so accommodate more than one smaller company.

It also has to compete with a downturn in the office space market - 18 per cent of London's office space is empty - and cheaper rents in Canary Wharf. It is estimated Swiss Re is losing £35,000 a day in lost rent, £5m since it opened.

But there is no doubt the public loves the Gherkin: 8,000 people queued for five-and-a-half hours earlier this month to get a glimpse of the inside, and the stunning views of the capital, during London's annual Open House Day.

The 40-storey, 590ft conical glass tower was built on the site of the Baltic Exchange, destroyed by the IRA in 1993. A distinctive feature are its 5,700 diamond-shaped and triangular glass panels which form a swooping spiral effect along its flanks.

In 1998, Lord Foster won the Stirling Prize, awarded annually for the greatest British contribution to contemporary architecture, for the American Air Museum in Duxford. Other past winners include the Gateshead Millennium Bridge and, last year, Herzog and de Meuron's Laban Dance Centre in Deptford, south-east London.

LANDMARK CONTROVERSIES

City Hall

Designed by: Foster and Partners

Where is it? On the south bank of the river Thames, near Tower Bridge

Opened: July 2002

What is it: Home to the Mayor of London, Ken Livingstone, and the Greater London Authority

Claim to fame: The GLA has praised the striking building which it moved into two years ago, as "a new landmark for the capital"

Critics say: "It's the most preposterous building of them all, and the one with the least elegance. It looks like a cheap children's party entertainer trying to juggle too many plates" (Laurence Llewelyn-Bowen, presenter of BBC1's Changing Rooms)

Selfridges, Birmingham

Designed by: Future Systems

Where is it? The Bull Ring, Birmingham city centre

Opened: September 2003

What is it? Futuristic department store inspired by a Paco Rabanne dress and covered in a skin of 15,000 spun aluminium discs with no traditional shop windows or external branding

Claim to fame: "Soft and curvaceous in response to the natural curve of the site." It has "re-interpreted the notion of a department store", say its designers

Critics say: "One of a generation of buildings which appear like giant pupae, laid by insect patients" (Brian Sewell, art critic and pundit)

Scottish Parliament

Designed by: Enric Miralles

Where is it? Holyrood, at the foot of Edinburgh's historic Royal Mile

Opened: October 2004

What is it? £431m flagship of Scottish devolution, completed three years late and 10 times over budget

Claim to fame: At its opening, Scottish Parliament Presiding Officer George Reid said: "We've got a work of art. The challenge now is to turn [it] into a working parliament"

Critics say: "Hideous. It won't date well, and for that price we should have been able to come up with something that looks a little less like a shanty town" (Sarah Beeny, presenter of Channel 4's Property Ladder)