Snow said on Friday that disclosing the sources and methods used by the programme could "only help the terrorists" and made the government's job more difficult.

The programme, run by the CIA and overseen by the treasury department, gave the US government access to millions of international financial transactions.

Treasury officials obtained access to an extensive international financial data base - the Society for Worldwide Interbank Financial Telecommunication, or Swift - following the September 11 attacks.

The White House and Treasury department issued statements on the programme after The New York Times, the Los Angeles Times and The Wall Street Journal posted stories about the programme on their websites.

The revelations drew protests from Democrats in Congress, who said it raised concerns about intrusions on privacy fom an aggressive Bush administration expansion of executive-branch powers.

Snow defended the programme, saying it was "responsible government, it's effective government, it's government that works.

"It's entirely consistent with democratic values, with our best legal traditions."

He said Congress had been briefed on the programme and that it contained safeguards to ensure the information was only used for terrorism investigations.

Snow said: "By following the money we've been able to locate operatives, we've been able to locate their financiers, we've been able to chart the terrorist networks and we've been able to bring the terrorists to justice."