Longs Drug Stores could come under pressure from activist hedge fund investor Bill Ackman to seek a higher takeover offer, either from current suitor CVS Caremark or from rival drugstore operator Walgreen.

Longs, a well-known drugstore chain in the western US, announced this week that it had agreed to be bought by larger rival CVS Caremark for $71.50 per share, in a deal valued at $2.9bn, including debt.