Was the attack on the World Trade Centre on September 11th, when hijacked planes were flown into the Twin Towers in the space of 20 minutes, one single insurable incident or two? The answer is worth $3.5 billion (€4 billion).

The main leaseholder of the property, Larry Silverstein of Silverstein Properties, argues the attack constituted two separate losses, entitling the insured to twice the $3.5 billion insurance cover. Swiss Re, the biggest insurer of the trade centre, has asked the Manhattan federal court to rule the attack was one loss instead of two separate losses.

A complication arises from the fact that there was no insurance policy on the Trade Centre in place when it was attacked. A group of investors headed by Mr Silverstein had taken over the management of the property in August and the wording of a new insurance policy was still under negotiation. The 22 insurers led by Swiss Re have agreed to be bound by the terms of the proposed policy but dispute the contention of two separate insurable incidents.

Real estate developer Mr Silverstein said he needed the insurance to construct new buildings on the site and compensate for lost rents. Including damaged buildings, 60 per cent of the financial district's modern office space - built after 1970 and outfitted for the latest technology - is unusable.