IBM has filed a counterclaim against the SCO Group, which is suing IBM for $3bn (£1.86bn) for allegedly breaking its Unix contract by allowing its code to be added to Linux.

Big Blue's filing claims that SCO has violated the GNU Public Licence (GPL) and four of IBM's patents, SCO told vnunet.com.

SCO confirmed that it had only just received the filings and said it was unable to comment further.

The GPL lays out the conditions under which the Linux operating system and other open source software may be used.

The filing was made in a Utah court late last night. SCO is based in Utah, and had made its filings in the Utah State Court.

On Monday, Linux distributor Red Hat also filed a countersuit against SCO, which fellow distributor SuSE was quick to support.

SCO has since issued a licence aimed at charging customers for using the Linux operating system which, it claims, contains its Unix code.

Market watchers said the action was bad news for SCO.

"Unlike SCO, IBM has been very careful not to make inflammatory statements. So when IBM fires a broadside it is a very carefully prepared action, which will be very carefully executed," said Gary Barnett, principal consultant at Ovum. "If SCO is wrong, IBM is absolutely right to ask for compensation."

"IBM has more patents than anyone, and more money and more lawyers than SCO. SCO will be bled dry before it can make its case," he added.

Mike Davis, senior researcher at Butler Group, commented: "This is going to be very interesting. SCO still hasn't declared what bits of code are offending. It's a very risky strategy [SCO] is pursuing.

"The other partners in the UnitedLinux consortium, especially SuSE, are reaching the stage of frustration with SCO. The SCO case is starting to put some hesitancy into users who are asking more questions, although it hasn't hit revenues yet. It is not helpful."

IBM declined to comment at time of going to press.