Willis has rejected claims it failed to complete paperwork for insuring the World Trade Centre (WTC).
The allegations, published in a Sunday newspaper, said the world's third-largest broker had not finalised contracts for the twin towers.
But a spokesman for Willis said coverage was in place in July and insurers were bound to the risk.
“Following customary industry practice, we continued working with the markets to finalise the detailed policy language after that date. The documents were signed off by the client and we issued them on Monday,” he said.