Lloyd's has joined other London-based insurers in the legal battle over the insurance of the World Trade Centre (WTC).
Twenty syndicates were asked to submit a supporting document to an appeal by US insurers Travelers, Hartfield, Royal and St Paul.
Lloyd's underwriters filed a brief with a US court on Friday, saying the attack merited one event.
Lloyd's is not part of the appeal but the document relates to its part in the Swiss Re v Silverstein court case, over whether the destruction of the WTC should be treated as one insured event or two events.
The filing supports the insurers' in their fight against WTC leaseholder, Larry Silverstein, who claims the attacks, were two separate events.
A Lloyd's brief told Reuters: "Silverstein parties valued the full replacement cost of the insured location at approximately $3.945bn (£2.3bn).
"At the same time, they chose to insure the entire property as one insured location for $3.546bn (£2.2bn). Now, they have hired litigation experts and developed a legal theory designed to create coverage for twice that amount against a single, coordinated terrorist attack destroying that single insured location."
The brief reportedly argued that the number of claims should be governed by the fact they were part of a single plan to bring down the towers.
The brief was understood to have claimed that Silverstein's approach would yield "absurd results" if adopted as law. He claimed that a large gang of terrorists simultaneously setting off bombs on each level of the World Trade Centre would have allowed "hundreds" of claims.
The brief was filed on behalf of 20 Lloyd's syndicates and specialist Lloyd's vehicles such as Amlin.
