Shell is having a tough time convincing investors to accept its 725p per share offer for Enterprise Oil and many are holding out for a higher offer, sources say.
Royal Dutch/Shell has been in the market for Enterprise shares since announcing a £3.5 billion sterling bid for the group on Tuesday.
But it had only managed to buy 11.7 per cent of the share capital by today in a sign that many were reluctant to sell up just yet.
Shell's 11.7 per cent, which includes 0.5 per cent held by the group's pension fund, falls far short of the 30 per cent that bidders are allowed to buy in the market under takeover rules.
Although hopes for a counteroffer faded yesterday when Italian energy giant Eni said it would not bid, Enterprise's share price still suggests some are hoping for a rival suitor to emerge.
France's TotalFinaElf or Norwegian Statoil are still seen as potential candidates, and industry observers did not rule out interest from North American concerns such as Chevron or Conoco.