Bermuda-based energy mutual Oil Insurance Ltd. has been dealt a blow by Standard & Poor's, who put the company's A- financial strength rating under review with negative implications.

OIL announced last week that week that 12 shareholder members are withdrawing from the structure, and one member will be ineligible for membership, as of June 1.

That brings the total of shareholder members to leave this year to 22, after nine departed in January, and it will leave OIL with 61 shareholder members.

"In Standard & Poor's opinion, the number of shareholders who have exited OIL in the past six months is significant. This raises concerns about the stability of the company's membership base and the strength of its competitive position in its chosen niches," the rating agency said in a statement.

"In the past year, OIL has made significant efforts to address the different needs of its membership base, which contributed to the company announcing an amended rating and premium plan that was approved by the shareholders in March.

"The combination of the recent exits and the amended rating and premium plan could lead to changes in the prospective volatility of OIL's insured portfolio that need to be evaluated."

S&P's credit watch on OIL is expected to be resolved within three months.

"In the event that following its review Standard & Poor's decides to lower the ratings on OIL, such a downgrade is not expected to exceed one notch," S&P added.