Fueling the price rise in Mercury International Group PLC shares was a report that New York investor Saul Steinberg plans to increase his stake to more than 15% from the 10.44% that he currently controls through one of his companies. Last November, Steinberg agreed to limit his stake to 10.44%. He disclosed his plan to increase the stake in a routine antitrust filing in Washington. Mercury shares closed in London at 3.60 pounds ($5.12 U.S.), up from 3.35 pounds ($4.76) on Wednesday. The shares closed at 3.20 pounds ($4.55) on Tuesday, before the announcement. Mercury owns S. G. Warburg & Co., one of Britain's biggest merchant banking houses.