Converium Holding Ltd. received the credit rating upgrade it had sought for more than two years, boosting the Swiss reinsurer's defence against a hostile takeover attempt by French rival Scor SA., Bloomberg News reported.

Standard & Poor's increased the financial strength rating to ''A-'' with a stable outlook from ''BBB+,'' the rating company said. It would consider lowering the rating should Scor's bid succeed and ''potential integration risks'' such as the loss of key Converium staff or clients would materialize, S&P also said.

''This gives Converium a further argument'' to fend off Scor, Lucio di Geronimo, an analyst at HVB Group in Munich, who rates the shares ''hold", told Bloomberg. ''There could be an increase in the offer price.''

Converium yesterday said it will double its payout to investors and promised to return $300 million more after a rating upgrade. Chief Executive Officer Inga Beale wants shareholders to stay with Converium rather than take an offer from Scor that values it at 3.1 billion Swiss francs ($2.5 billion).