For the first time since 1987, the largest shipowners' mutual insurance group will not be raising liability insurance rates for members.

Stephen James, director of Thomas Miller P&I, which manages the United Kingdom Mutual Steam Ship Assurance Association (Bermuda) Ltd., said the mutual's board reflects a flat period for claims and the club's much stronger

financial position after several years of very large increases in premiums.But Mr. James said he does not expect the slowdown in claims to last for long. The U.K. Club is the largest protection and indemnity club, with about a quarter of the world's merchant fleet on its books.

Mr. James made the announcement while commenting on the decision by Lloyd's of London this week to accept corporate membership.

He said the injection of fresh money into Lloyd's, the world's largest marine insurance market, should expand the amount of reinsurance capacity available to the world's P&I clubs after several years of contraction.

It also will make negotiations for next year's reinsurance coverage easier, he said. For now, the club has $1.043 billion in hand against estimated claims of $1.028 billion.

The 18 members of the International Group of Protection and Indemnity Clubs, which account for around 90 percent of the global shipowner liability market, buy their reinsurance coverage together through a single contract.

But in the past two years the clubs have been unable to obtain as much reinsurance as they wanted in London and have placed some of the business elsewhere, especially in Bermuda, where the marine reinsurance market is growing rapidly. Even so, the clubs were unable fully to cover their requirements.

At the same time, the cost of the clubs' reinsurance contract at the February 1993 renewal date soared by 130 percent compared with the previous year, Mr. James said.

Although he does not expect marine reinsurance premiums to decrease as a result of the extra underwriting capacity available to Lloyd's from next year, Mr. James said brokers should have a better chance of placing the clubs' reinsurance contract in full.

He cautioned, though, that much would depend on the reinsurance market's reaction to the recent Tampa Bay oil spill and Alabama railway disaster.

Each of the shipowner-owned clubs pays the first $3 million of any claim. This amount soon will be raised to $5 million.

Claims up to $25 million are pooled between the clubs, while any claim in excess of that amount is covered by reinsurance up to a limit of just over $1 billion, after which claims are shared again.

Shipowner mutuals traditionally charge supplementary calls during the policy year rather than charging the whole of the premium at the start of the period to ease their members' cash flow.

The planned supplementary call for the 1992 policy year has been reduced

from 40 percent to 25 percent because of the lower-than-expected level of claims.

However, a 40 percent supplementary call still is planned for 1994, even though there will be no general increase in premiums at the beginning of the policy year.