Home secretary Jacqui Smith has been summoned to the Commons to explain how a second sensitive security document was found on a train less than a week after the last one.
The second set of papers - covered drug trafficking, money laundering and the financing of international terrorism - were found on a train heading to Waterloo station.
The endless stream of security breaches coming from the government has led many campaigners to say it is incapable of running its controversial ID cards scheme. Last Wednesday, David Cameron accused the government of "ineffective authoritarianism".
Foreign secretary David Miliband said the security lapses were "completely inexcusable".