The documents were found on a train heading to London's Waterloo station
More sensitive government files were found on a commuter train earlier this week, it has been revealed.
The Independent on Sunday says it was handed the documents, which cover fighting global terrorist funding, drugs trafficking and money laundering.
The files were found on the same day as the BBC was handed top secret papers on al-Qaeda. A Treasury spokesman said the government was "extremely concerned".
The Tories are calling for controls to protect secret official information.
The documents, about a meeting of financial crime experts, apparently include briefing notes for a meeting of the international Financial Action Task Force (FATF) to be held in 11 Downing Street next week.
The papers were found on train bound for London Waterloo on 11 June, the same day that another batch of papers relating to intelligence assessments of Iraq and al-Qaeda were handed to the BBC after being left by a senior official on a train.
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.
BBC political correspondent Laura Kuenssberg said it was uncertain whether the latest documents were also top secret.
The documents seen by the BBC should not have left Whitehall but it is not yet clear if the new files were permitted to have been taken out, our correspondent added.
"Some of the information is already on the public domain, but another lapse is deeply embarrassing for the government," she said.
A Treasury spokesman said: "We are extremely concerned about what has happened and we will be taking steps to ensure that it doesn't happen in the future."
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 government needs to get a grip in order to protect this sort of sensitive information and the British public
Dame Pauline Neville-Jones
Tory shadow security spokesman
Chairman of the Home Affairs Select Committee, Keith Vaz, said people would be "alarmed" at this latest revelation.
He said that until an inquiry had established how the leaks happened, "no official no matter how senior, should be allowed to take classified or confidential documents outside their offices for whatever reason.
"Our enemies don't even need to hack into our computers, they apparently just need to travel on public transport."
Dame Pauline Neville-Jones, Conservative shadow security spokesman, said: "We've now had eight major breaches that we know of in six months.
"The government needs to get a grip in order to protect this sort of sensitive information and the British public."
She called for "cleared and trusted" supervisors appointed to "supervise handling of government information inside the machinery of government on a daily basis".
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.
It has since expanded to 34 members.