Shares in Royal & SunAlliance (R&SA) closed up by almost 5% at 81 1/2p on Friday after reports that a UK entrepreneur had planned a bid for the company.
UK financier Andrew Regan is reported to have begun targeting R&SA at the beginning of the year.
Regan is yet to approach R&SA with a bid, but it is understood that he flew his team to New York last month to arrange financial backing.
Pressure is mounting on the entrepreneur to make his intentions public after shares in his investment company, Corvus Capital, rose 30% to 10 3/4p.
More than 80 million R&SA shares were exchanged on Friday, considerably higher than the usual rate.
R&SA declined to comment.