Ascot Underwriting's Syndicate 1414 leads the syndicates at Lloyd's which covered the UK's biggest cash robbery in Tonbridge.
The gang has stolen £53m from a Securitas depot and police say they are closing in on them.
Business development manager Torquil McLusky said: "If, as has been reported in the press, the losses exceed £20m then we will take a hit, as will the excess layer."
The cover was placed under the specie account as cash in transit, with a captive insurance first layer and excess coverage led by Millennium Syndicate 1221.
McLusky added: "This was renewal business for us and Securitas is a good client with the right attitude to risk management. Nobody wants to pay a claim, but, while it is a line, that's what we're here for. With other insurers, we have put up a £2m reward for information leading to the recovery of cash and conviction." Marsh acted as broker for Securitas.
The theft occurred after the manager of the depot and his wife and son were kidnapped.