Employees of Swiss Re were contemplating a sweltering summer this week after it emerged that half of the 'Gherkin's' 700 windows had been sealed up.

The windows of 30 St Mary Axe, which could previously be opened to provide extra ventilation, were permanently closed after one of its giant windows fell 400ft to the ground last month.

A spokesman for the property said: "The building has a traditional air-conditioning system and over half of the windows can still be opened. This will in no way affect the working environment here."