MI5 concerns over The Guardian sending top secret files... by Fedex: Newspaper used public courier firm to post data out of the country The newspaper saved files from leaker Edward Snowden to a USB stick It then sent the files to the USA via the FedEx delivery service The move has been described as creating a security risk Intelligence experts have accused The Guardian of 'playing parlour games' with national security By Robert Verkaik The Guardian newspaper sent tens of thousands of top-secret files containing the names of British spies to the United States by household courier FedEx, it was revealed last night. The security risk is the latest to engulf the newspaper after the head of MI5 warned last week that publication of confidential data leaked by US fugitive Edward Snowden had caused huge "harm" to the capability of Britain's intelligence services. Last night, senior security figures lambasted the newspaper for using Federal Express to carry the encrypted files. 'Huge damage': The leaks from former intelligence worker Edward Snowden, pictured here received an award while in exile in Moscow, have been condemned by intelligence experts Former anti-terror law watchdog Alex Carlile QC said the decision may have put the lives of British spies in danger. "I think it is astonishing that top-secret material has been placed in a public parcel delivery system," he said. Security chiefs are also understood to be "very concerned" at the use of a courier company. One source said: "Any leak like this is extremely concerning and part of that concern is the handling arrangements." The data - from GCHQ, the Government's secret communications headquarters - was copied from the original Snowden cache on to a USB memory stick and sent by FedEx to a "disguised" US intermediary in June this year. Measures were taken to ensure its safe passage. He was told that if The Guardian was "muzzled" he was to make sure the material was published on ProPublica, a not-for-profit New York website which publishes investigative journalism. Weeks later, GCHQ officials visited the newspaper's London offices to destroy computers and hard drives in the belief they had closed the security breach. 'Parlour games': The Guardian, edited by Alan Rusbridger, pictured, has been accused of endangering MI5 and MI6 agents The decision to send the files by courier is believed to have been taken within days of Guardian editor Alan Rusbridger returning from America, where he had been working with the Guardian's New York office on how to handle the fall-out from the Snowden leaks. Mr Rusbridger has since admitted he was worried the Government would seek an injunction to seize the material, preventing any further publication of the Snowden files. According to a report last week in The New Yorker, Mr Rusbridger sent a "Federal Express package containing a thumbnail drive of selected Snowden documents to an intermediary in the US." The magazine told The Mail on Sunday that it stood by its story, which had been "multiple sourced." The Government only became aware that material may have been sent out of the country when Rusbridger met Cabinet Secretary Sir Jeremy Heywood and the Prime Minister's head of communications, Craig Oliver, at The Guardian's offices on June 21. According to The New Yorker, Rusbridger hinted that the Government might be too late to prevent publication of further material. On July 20, The Guardian gave permission for officers from GCHQ to destroy hard drives holding the secret files at its King's Cross offices. Vulnerable: The Guardian reportedly uploaded a cache of intelligence files into a USB memory stick to send them to the USA Two days later, The Guardian admitted to the Government that "secret material" had been given to ProPublica and the Wall Street Journal. It is not clear whether The Guardian told the intelligence services that the material had been sent by FedEx, when it was sent or where from. Last night, Stephen Engelberg, editor-in-chief of ProPublica, confirmed he had received the files. He said: "The material is in our possession. It's in a safe place. At one point we were told the Crown was going to ask for it back but they never did. He confirmed the memory stick "was sent" but when asked how or from where, he said: "I'm not going to go into that... It came through. 'I don't know how it got to me. It found its way through. We were prepared had they been injuncted to do what needed to be done. Last night, the revelations about the sending of secrets was branded "astonishing" by Lord Carlile, who called on the Attorney General to consider legal action against the newspaper. Lord Carlile accused The Guardian of playing "a parlour game" with national security. His comments are at odds with other leading Lib Dems, including Business Secretary Vince Cable, who have supported The Guardian. Special delivery: The Guardian used the Fedex parcel service to deliver the files He said: "I am very surprised The Guardian has not paid enough attention to comments by Andrew Parker [MI5 director general] and others about the damage these disclosures have done or can still do to national security. There are individuals whose safety might be affected by these disclosures. We are talking about MI5, MI6 and the national security agents of other countries. I think it is astonishing that a large amount of top-secret material has been placed in a public parcel delivery system. I think it is quite wrong that it should have been sent outside the country to avoid the jurisdiction of the UK courts. I hope the Attorney General will examine what The Guardian has done to determine whether there is any action to be taken. National security is not a parlour game. Attorney General Dominic Grieve could seek an injunction against The Guardian to deliver up the material. If refused, the editor would be in contempt of court and could be imprisoned for up to two years. Last night, The Guardian refused to say how the material was sent. A spokesman said: "An encrypted form of a limited number of documents was safely sent to ProPublica. We're not going to talk about what those documents were. In 2010, FedEx was at the centre of the printer bomb alert when an Al Qaeda bomb was found at its depot in Dubai. And in 2006, a former employee admitted selling confidential account numbers of clients using FedEx's London depot. Snowden's treachery has put the lives of our spies at risk: Comment by Richard Kemp, ex-chairman of COBRA intelligence committee