Fidelity, the global investment house, has been a heavy seller of shares in BET, the industrial services group facing a hostile pounds 1.9bn bid from Rentokil.
The firm, the second largest shareholder in BET before the bid was announced last month, has reduced its stake from 5.17 per cent to about 3.3 per cent.
Mercury Asset Management, which held 1.7 per cent at the start of the bid battle, is also thought to have been a steady seller.
Share traders have said there has been heavy demand from the United States as arbitrageurs piled into BET in the hope that Rentokil will increase its offer.
One analyst said that the move by the two investment firms suggested some doubts that Rentokil would increase its offer significantly.
"There seems to be a lot of hedging of bets here. BET had been a dog and it makes sense to cash in a few shares," the analyst said.
The offer is nine new Rentokil shares and pounds 8 cash for every 20 BET shares, valuing BET at 190p a share at the start of the bid.
Meanwhile, BET is not expected to publish its offer document until the middle of next week at the earliest.