"Lloyd’s has detected unusual activity on its network and we are investigating the issue," a Lloyd's spokesperson said by email, adding that the market was resetting the network.
"We have informed market participants and relevant parties, and we will provide more information once our investigations have concluded."
Lloyd's, which has been vocal in its support for sanctions against Russia, declined to comment on the attack's possible source.
At Lloyd's, approximately 100 insurance syndicates operate.
Several of the syndicates provide cyber insurance to protect against such attacks.