In a letter to shareholders released yesterday, Argus said Tweed's Direct Share Purchasing Corp had written to BHP Billiton requesting a copy of the company's share register, something the company is obliged to provide.

"You may know of [Tweed]; he is mentioned frequently in the media for making offers to the holders of small parcels of shares in listed companies," Argus said.

"It concerns me that shareholders may unknowingly accept an offer for their BHP Billiton shares that substantially undervalues their shareholding."

Argus warned shareholders to check the value of any offers received against the company's current market value via either the Australian Stock Exchange's website or daily newspapers, and to check how quickly payment would be made for any shareholdings.

Tweed has picked up discounted interests in numerous listed companies by making below market value offers to shareholders after obtaining a company's share register.

Shares in BHP Billiton were up 14c to $27.87 in midday trade today.