Marsh will be under pressure from other London market brokers following the decision of Zurich Specialties London to cease underwriting US domiciled liability business.
A market source said Marsh was the broker for "almost half" of Zurich Specialties' US casualty book, which is understood to be £100m in size.
The source said: "Other brokers will be looking to pick up the business from Marsh on renewal."
Zurich Specialties is understood to have ceased underwriting the US business in order to provide capital to boost its ratings.
A spokeswoman for Zurich Specialties London confirmed the company had ceased to write general liability for US domiciled customers. Marsh was unavailable for comment.