The London P&I Club has placed its reinsurance renewal, worth an estimated £150m, with Catlin Underwriting Agencies , according to market sources.
It is understood the deal is being finalised at present for the first and second layers of the reinsurance contract.
This covers liabilities for damage to piers, cargo and harbours and is one of the largest marine contracts.
One source said this was a "severe blow" to Limit Underwriting, as it meant the severing of a 50-year relationship with the underwriting team of Janson Green.
Limit was formed by the merger of Janson Green and Bankside Syndicates in 1999.
Market sources said Catlin won the business because it made "a better offer".
They said: "The P&I Club has a tremendous record in its claims. The premium is extremely high with very few claims made over the years. "
Shipowners were prepared to continue placing business with an underwriter who would look after them in the future, they said.
"But with the market today, this cannot be guaranteed," they added.
Speculation that the premium could be as high as £400m was dismissed as "slightly outlandish" by the sources
A spokeswoman for QBE Insurance said: "Limit is unable to comment on this issue at this time." No one at Catlin Underwriting Agencies was available to comment.
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