Britain’s High Court has upheld a decision that the September 11 2001 attacks on the World Trade Centre Twin Towers were two events.

It comes after reinsurer Aioi Nissay Dowa Insurance took cedant reinsurer Heraldglen – managed by run-off company RiverStone – and Lloyd’s syndicate member Advent Capital (No. 3) to court, arguing the attacks were one event.

The litigation stemmed from claims against American Airlines and United Airlines, whose planes were flown into the New York towers, and security companies that screened passengers.

Heraldglen and Advent settled claims on the basis the attacks were two events, then made excess-of-loss claims against Aioi on that basis.

The High Court hearing was an appeal of an arbitration tribunal ruling that the attacks were two events.

The tribunal decided that, although the Twin Towers were close to each other and part of one property, the attacks did not constitute one event.

It ruled there were two hijackings of two separate aircraft. If only one hijacking had succeeded, only one tower would have been destroyed.

The tribunal used the final report of the National Commission on Terrorist Attacks Upon the US in reaching its determination, but retrocession excess-of-loss covers were governed by English law.