Blunders involving sensitive official information have hit the headlines
The Conservatives have accused the government of presiding over "a culture of carelessness" after a second set of sensitive papers was left on a train.
The files - on terrorist funding, drugs trafficking and money laundering - were handed to the Independent on Sunday.
Dame Pauline Neville-Jones, shadow security spokesman, said those responsible for handling confidential information were "flouting" the rules.
The foreign secretary told the BBC the incident was "completely inexcusable".
The documents were found on train bound for London Waterloo on 11 June, the same day that a batch of papers classified "UK Top Secret" and relating to intelligence assessments of Iraq and al-Qaeda, were handed to the BBC after being left by a senior official on a train.
Chairman of the Home Affairs Select Committee Keith Vaz said he would ask Home Secretary Jacqui Smith to appear before the committee next week to explain whether either blunder could hamper the country's fight against terrorism.
Mr Vaz said: "Until the inquiry has been concluded and we are told how these extraordinary events occurred, no official, no matter how senior, should be allowed to take classified or confidential documents outside their offices for whatever reason."
'Utmost seriousness'
The foreign secretary confirmed there would be a thorough investigation into the latest incident.
"The government takes all reports of security breaches extremely seriously. These sorts of lapses are completely inexcusable," David Miliband said.
It's frankly an appalling blow to the government's efforts to tackle financial flows underpinning both organised crime and terrorism
"I know that this will be taken with the utmost seriousness and will be followed up to ensure that there isn't a further threat to the national interest."
Earlier a Treasury spokesman said the government was "extremely concerned" about the lapse in security and would be "taking steps to ensure that it doesn't happen in the future".
Some of the documents, about a meeting of financial crime experts, were said to have included publicly available material.
They also the apparently include briefing notes for a meeting of the international Financial Action Task Force (FATF) to be held in 11 Downing Street next week.
Dame Pauline said it was "yet another example of a lapse in discipline" by the government.
"I think it is a culture of carelessness," she said.
"People taking rules in departments - and all departments do have rules on the whole question of the protection of government information - not taking them seriously and flouting them."
Dame Pauline called for greater controls to protect secret official information.
"We have to have much more long-term, independent governance and supervision and investigation of the custody of sensitive information.
"In other words, we have to have people who are appointed by government who are trusted, who actually supervise the handling of government information inside the machinery of government on a daily basis."
The new Shadow Home Secretary Dominic Grieve said there was "a systems failure going on".
Chris Huhne, Liberal Democrat home affairs spokesman, said the incident was "deeply embarrassing".
Keith Vaz said secret documents should not be taken out of the office
"It's frankly an appalling blow to the government's efforts to tackle financial flows underpinning both organised crime and terrorism and it really shows us up as the chair of international efforts to deal with this problem."
Documents returned
The Cabinet Office and the Metropolitan Police launched inquiries into the documents handed into the BBC - the latest in a series of blunders involving sensitive official information.
But Scotland Yard said it was not involved in investigating the latest case. The Independent on Sunday said it had returned the documents and would not be divulging any details contained in them.
The confidential files were said to include details of how trade and banking systems could be manipulated to finance illicit weapons of mass destruction in Iran.
They also discussed methods of terrorist funding and the potential fraud of commercial websites and international internet payment systems.
The FATF conference is due to begin on Monday at the QE2 Conference Centre in Westminster, opposite the Houses of Parliament.
The FATF was established by the 1989 G7 summit in Paris to spearhead efforts to counter the use of the international financial system by criminals.
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