UK passengers travelling to the US could have their credit card transactions and e-mail accounts inspected by officials.
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An agreement between the European Union and Washington struck in October gives US officials access to detailed passenger information.
A document released under the Freedom of Information Act reveals they could now access bank and e-mail accounts.
Critics say the move is an infringement of people's right to privacy.
Legal channels
The document, released by the Department for Transport, outlines the undertakings of the US Department of Homeland Security after the October agreement.
Under the agreement, US officials can request 34 separate items of Passenger Name Record (PNR) - which airlines must hand over.
The document, released after a Freedom of Information request by the Daily Telegraph, then says US officials could use legal channels to obtain further information from the details given.
This could include transactions linked to a credit card account or records related to an e-mail account, the document says.
It means that anyone using a credit card to pay for flight tickets or giving their email address to an airline could be affected.
'Worrying'
The document says PNR data will be used strictly to prevent and combat terrorism and other serious crime, including organised crime.
The Conservative Party described the agreement as "worrying" while campaigners said it eroded people's rights to privacy.
Shadow Transport Secretary Chris Grayling said: "This agreement is extremely worrying.
"It seems to be completely one sided and gives the US administration very significant powers to access private information from British air passengers.
"It is wrong that the deal was not made public from the start, and that it has taken the Freedom of Information enquiry to discover what is going on."
'Infringement'
Simon Davies, of Privacy International, said: "The Europeans have capitulated to the Americans because they fully intend doing exactly the same in Europe over the next few years.
"They will claim it is international standard. It is an infringement of people's right to privacy since after the information is handed to the US government you have no control over it."
A spokesman for the Department for Transport said: "The whole of the EU has signed up to the agreement. It is not just UK passengers.
"An alternative would be simply having no European flights going to the US. It is up to the US authorities to decide what they do with the information."
