Cracks take shine off Foster's erotic gherkin

It's one of the most talked-about buildings in London. The cigar-shaped frame - or "erotic gherkin", as it has been dubbed - dominates the City skyline. Swiftly erected and brilliantly lit from the first, the dazzling curving steel diagrid exoskeleton has been hailed as a triumph of design and engineering. Next month, the first office workers are due to move in. But there's a problem - some of the glass panels on Lord (Norman) Foster's innovative Swiss Re tower have cracked.

With just four weeks to go until the final completion date of the 40-storey tower, engineers are racing against time to replace a number of mysteriously cracked panes in the celebrated glass exterior.

The discovery of the cracks - believed to affect at least 15 of the giant tinted panes - has caused unwanted problems for Zurich-based owners the Swiss Reinsurance Company (Swiss Re) on the eve of the building's official unveiling.

The main contractor, Skanska Construction, which won the £130m job of erecting the tower, claims the cracks have nothing to do with the structural integrity and probably occurred during the manufacturing and transportation of the glass. But they are at a loss to explain why these broken panes were then fitted.

"Damaged panels were recorded as part of our quality-control procedure," said a spokeswoman for Skanska. "As a matter of course, a safety check is carried out to ensure that the integrity of the panel is not compromised. No panels have cracked after their installation."

Skanska's project manager, Richard McNaughton, was less clear about what exactly had gone wrong. "I can't be sure when it happened," he admitted. "If cracks were seen during the installation, they would have been dealt with at the time, and the same goes for the manufacturing stage."

Mr McNaughton was confident that the panes - along with eight others that had inexplicably been fitted with the wrong glass - would be replaced in time for the first wave of staff to move in as planned on Monday 15 December.

The tower, which has taken nearly three years to complete, contains over 5,700 glass panels. Swiss Re plans to occupy 14 storeys, with most of the remaining floorspace being rented out to other companies. But none has been let so far.

It is not the first groundbreaking glass-rich building in the capital to have problems with materials. Eurostar, the owner of the £120m Waterloo International station, had to replace panels in Sir Nicholas Grimshaw's award-winning structure when safety experts discovered that some of the glass roof was in danger of exploding. The MPs' new office block, Portcullis House, designed by Michael Hopkins and Partners, suffered from glass panels cracking while Lord (Richard) Rogers's Thames-side development Montevetro - "glass mountain" in Italian - required 1,000 panes to be removed and resealed because they were leaking.

Lord Foster, the "blue-chip world's favourite architect", has also encountered difficulties in the past. At his £12bn Chek Lap Kok airport in Hong Kong, parts of the terminal's 70ft-high glass walls had to be replaced after mysterious bubbles were found in some panels and others looked as if they had had bricks thrown at them.

Yesterday, experts declared themselves "mystified" by how the cracks had appeared.

"If the panes were damaged during transportation, why were they installed?" said Tom Cullen of the trade publication Construction News. "If they were damaged during installation, why weren't they removed? Some of these panels are extremely sizeable, and the ones at the top are entirely different shapes. It's going to be extremely pricey per crack."

Lord Foster's office declined to comment last night. A spokeswoman for Swiss Re declared that staff would be moving in as planned.