Oil company Nighthawk Energy today said it had uncovered the identity of online investment chatroom users it claims have been spreading "defamatory and untrue" rumours.

The group - listed on the Alternative Investment Market (AIM) - went to court to force websites ADVFN and iii to reveal the identities of the anonymous bulletin message posters.

The High Court ruled in late June that the postings could be defamatory and granted the order, according to Nighthawk.

It is now considering legal action against the individuals, whom the company alleges made "persistent" postings about the firm's operations and its management "which the board considers to be extremely damaging to the company and its reputation".

Nighthawk added: "The company took steps to identify the posters who try to escape liability for their defamatory postings by hiding behind a cloak of anonymity, using anonymous email, social networking or bulletin board accounts to disseminate those postings.

"In some cases the names and other details used to register these accounts are fictitious and posters also can try to cover their tracks using numerous pseudonyms, but they can be traced through their IP addresses."

Fellow AIM-listed oil stock Nostra Terra Oil and Gas also confirmed it was considering similar action after claims were made about the group's management in bulletin board postings.

Chief executive Matt Lofgran said he and the firm had been the target of postings since just before the start of the year.

The firm is also looking at potential legal action after recently seeking a court order to unmask the chatroom users.

Internet bulletin boards attract thousands of postings each day from retail investors discussing stocks and their portfolios.

ADVFN said it receives between 10,000 to 12,000 postings a day and has received several court orders already this year requiring it to reveal the identities of users.

The group said its policy was not to police the bulletin boards and does not reveal user identities unless served with a court order.

But it said it will remove certain postings if complaints have been received and always complies with court orders.

It has already been served with several court orders this year.

The Financial Services Authority (FSA) said bulletin boards were being looked at closely as part of its efforts to combat market abuse.

It issued a £15,000 fine in March 2005 to investor David Isaacs for internet market abuse, which saw him post dishonestly obtained information on an internet bulletin board to boost a company's share price.