A former employee of GCHQ, the signals intelligence agency, was charged yesterday with leaking details of an Anglo-American operation to eavesdrop on members of the United Nations Security Council in the run-up to the war in Iraq.

Katharine Gun, 29, who was sacked from her job as a translator with the agency, is accused of passing classified information to an unauthorised person under Section 1 (1) of the Official Secrets Act.

The charge follows the publication of an article in The Observer in early March disclosing a request from the Americans for GCHQ's help in intercepting diplomatic traffic to help predict the outcome of any vote on Iraq at the UN . It is understood Mrs Gun's lawyers will not dispute that she was the source of the article but will argue that she was justified in disclosing the information.

Mrs Gun, who was dismissed in June, said in a statement: "Any disclosures that may have been made were justified on the following grounds: because they exposed serious illegality and wrongdoing on the part of the US government, who attempted to subvert our own security services; and to prevent widescale death and casualties among ordinary Iraqi people and UK forces in the course of an illegal war.

"No one has suggested (nor could they) that any payment was sought or given for any alleged disclosures. I have only ever followed my conscience." Mrs Gun is being represented by the civil rights group Liberty, which conducted the defence of David Shayler, the former MI5 officer jailed for passing secrets to a newspaper .

The case will inevitably cast light on the Government's conduct in the run-up to the war, with Liberty hoping to extract details of the legal advice offered to the Cabinet by the Attorney General. But, as in the Shayler case, the terms of Section 1 require the prosecution to prove only that information was passed to an unauthorised person, regardless of motive.

The Government Communications Headquarters (GCHQ) in Cheltenham is by far the largest agency in Britain's intelligence community but there have been few prosecutions of staff. Its last great embarrassment was the discovery of the Soviet spy Geoffrey Prime in 1982.

Mrs Gun was arrested in March, shortly after the publication of the article, which claimed that GCHQ's sister organisation in the United States, the National Security Agency, had asked for a "surge" in eavesdropping on UN delegations from Angola, Cameroon, Chile, Bulgaria, Guinea and Pakistan, which were members of the 15-strong Security Council.

The memo requesting the surge was said to have been written by Frank Koza, defence chief of staff (regional targets) at the NSA.

Mrs Gun, from Cheltenham, was charged at the town's police station by officers of the Metropolitan Police Special Branch.

A spokesman for Liberty said: "It is clear that the Government must be uncertain about the wisdom of this prosecution. It has taken six months to decide to proceed to charge."

Shaun Williams, the director of corporate affairs for Guardian Newspapers, the parent company of The Observer, said: "The story we published exposed serious illegality and wrongdoing on the part of the US government. It was a story of enormous public interest at a time of extreme international tension which was followed up worldwide."

Mrs Gun was granted bail and will appear at Bow Street magistrates' court on Nov 27.

Katharine Gun, 29, of Cheltenham, was charged by Metropolitan police special branch officers under section 1 (1) of the act. The section states that any serving or former member of the security and intelligence agencies is guilty of an offence if they disclose "any information" about their work without official authority.

In a statement last night, Ms Gun said: "Any disclosures that may have been made were justified because they exposed serious illegality and wrongdoing on the part of the US government which attempted to subvert our own security services. Secondly, they could have helped prevent widescale death and casualties amongst ordinary Iraqi people and UK forces in the course of an illegal war."

She said no money was involved in any disclosure. "I have only ever followed my conscience," she said.

Ms Gun, a GCHQ translator, was arrested in March - more than eight months ago - at a time when it was reported that America's national security agency, the US equivalent of GCHQ, was conducting a "dirty tricks" operation.

The operation was directed against UN security council members as part of Washington's battle to win votes in favour of a war against Iraq, the Observer newspaper reported. The story, the paper said yesterday, "exposed serious illegality and wrongdoing on the part of the US government".

The secret surveillance operation involved intercepting the home and office telephone calls and emails of delegates to the UN.

The NSA made clear that the particular targets of what was described as an eavesdropping "surge" were the delegates from Angola, Cameroon, Chile, Bulgaria, Guinea and Pakistan - the six crucial "swing votes" on the security council.

A memo sent by Frank Koza, a senior NSA official, said the information would be used for the US's "QRC" - quick response capability - "against" the key UN delegations.

Yesterday, Ms Gun was accompanied at Cheltenham police station by her lawyer, James Welch of Liberty, the civil rights group which is supporting her case.

She was granted bail to appear at Bow Street magistrates' court in central London on November 27.

It is understood that another person has been questioned in connection with the alleged disclosure.