The government has expressed concern at a court ruling that the Real IRA is not an illegal organisation.

In a case heard on Wednesday, Mr Justice Girvan cleared four men of being members, insisting that an organisation is proscribed only if it is listed under current legislation, or operates under the name of a listed organisation.

"Schedule 2 of the [2000 Terrorism] Act does not include any organisation called or known as the Real Irish Republican Army," said the judge.

A spokesman at the Northern Ireland Office insisted that the government would appeal.

"The government is very concerned at this ruling and the DPP [director of public prosecutions] is forwarding a report to the attorney general with a view to appeal," he said.

"The government is clear that the RIRA should be a proscribed organisation."

"We thought that was reflected in the law and parliament also believed it was reflected in the law," he added.

"The judge in this case disagreed.  We will appeal, and if necessary after that we will change the law."

But Victor Barker, whose son James was one of 29 people killed in the 1998 Omagh bombing by the Real IRA, accused the ministry of "breathtaking incompetence".

"It is apparently a very technical argument as to why this organisation is not proscribed," he told BBC Radio Ulster on Thursday.

"It seems to me that someone in the NIO has shown a breathtaking incompetence to put us in the position where, clearly, a very active terrorist organisation is not proscribed under the legislation."