In a classified programme begun weeks after the 11 September terrorist attacks, counter-terrorism officials have had access to selective data sent over the Swift network, which routes about $6 trillion daily between banks, brokerages, stock exchanges and other financial services institutions.

The programme is run by the Central Intelligence Agency and overseen by the Treasury Department and is limited to tracing transactions of people suspected of having ties to terrorist networks. But unlike other searches where investigators must obtain court permission to examine records, the US government has used the president's economic emergency powers to subpoena tens of thousands of records from Swift.

According to a New York Times report, the records mostly involve wire transfers and other methods of moving funds across borders.

The US government also accesses and tracks ATM and other transactions from Western Union, but the Swift programme is thought to be the largest and most far-reaching attempt to trace terrorist financing.

Nearly 20 current and former government officials and industry executives discussed aspects of the Swift operation with The New York Times on condition of anonymity because the programme remains classified. According to the paper, some of those officials expressed reservations about the programme, saying that what they viewed as an urgent, temporary measure had become permanent nearly five years later without specific Congressional approval or formal authorisation.

Stuart Levey, an under secretary at the Treasury Department, told New York Times reporters that the programme provides "a unique and powerful window into the operations of terrorist networks" and is a "legal and proper" use of authorities.

In a statement Swift says in the aftermath of the 11 September attacks, it responded to compulsory subpoenas for limited sets of data from the Office of Foreign Assets Control of the United States Department of the Treasury.

"Swift negotiated with the US Treasury over the scope and oversight of the subpoenas. Through this process, Swift received significant protections and assurances as to the purpose, confidentiality, oversight and control of the limited sets of data produced under the subpoenas. Independent audit controls provide additional assurance that these protections are fully complied with," says the statement.