Data retention - the controversial plan to store traffic information about all personal and business faxes, phone and mobile calls for three years - has quietly become law after a last-minute amendment to a Bill on terrorism.

The late amendment to the Criminal Justice (Terrorist Offences) Bill, introduced by the Minister for Justice, Michael McDowell, received almost no debate in nearly empty Dáil and Seanad chambers and was easily passed.

"The fact that some members of the Oireachtas were unaware this had passed shows how surreptitious this process was," said Malachy Murphy, co-chairman of the Irish Council for Civil Liberties.

Many legislators, privacy advocates and business groups had been expecting a separate Bill on data retention and had no idea the subject was introduced as an amendment to another Bill.

For over two years, Mr McDowell had repeatedly promised a specific Bill on retention, after Irish and multinational businesses expressed concern at the proposal and negative public reaction.

He had stressed the need for broad and open Oireachtas and public debate.

In the past three years, Data Protection Commissioner Joe Meade repeatedly threatened the department with a constitutional challenge unless it introduced such a Bill, according to documents obtained under the Freedom of Information Act.

Mr Meade was concerned that the Government secretly introduced mandatory data retention for three years in May 2002 through a Cabinet direction. Under existing data protection law, data can be retained only for six months.

According to documents, the Department of the Marine, Communications and Natural Resources has repeatedly expressed concern to the Department of Justice at the proposed three-year period.

It fears the State's ability to attract inward investment could be damaged and that a three-year retention period would have negative personal privacy and civil rights implications.

Mr McDowell argued last month in the Oireachtas that the amendment had to be hurriedly introduced after Mr Meade again threatened the Government in January.

But leading international privacy watchdog Privacy International stated: "The Government has long been aware of the legal standing of retention in Ireland, and so should have rectified it at an earlier date. Instead, they waited for a strategic moment in 2005 [ and] concealed the policy in an old Bill just before it was to be passed, with practically no debate."