Ex-Welsh Secretary John Redwood opposed plans for compulsory national identity cards in the 1990s, newly released documents reveal.
In government papers from 1994, Mr Redwood said "any hint" of mandatory ID cards would cause "difficulties with nationalist and devolutionist groups".
He said the benefits were "not proven - even to the police and the courts", who were most likely to be "keen" on them.
The prime minister at the time, John Major, later shelved the proposals.
Conservative Mr Major had previously said the cards were key in the fight against crime.
Before the Welsh assembly was created in 1999, powers over matters such as the NHS, education, housing and agriculture were generally the responsibility of the secretary of state for Wales - giving the holder of that office far more power than today.
In the newly released cabinet papers, Mr Redwood, who served as Welsh secretary from 1993 to 1995, wrote to Home Secretary Michael Howard saying he had "reservations" about compulsory cards.
He said: "At a time when constitutional questions affecting Wales and Scotland are again coming to the fore, discussion of a compulsory ID scheme would add unnecessarily to the existing debate about the future of the union."
In an earlier letter to the prime minister he said: "A compulsory scheme would be unacceptable to very many people, and I can see little merit in a voluntary scheme which would only attract partial compliance. The costs of either scheme are extremely high."
Mr Major had admitted there were "great practical difficulties" in introducing compulsory identity cards, but maintained they could be useful in the fight against crime.
However, in 1996, Mr Major's government shelved the plans after a consultation revealed the public were split three ways on the issue.
The issue of compulsory ID cards later re-surfaced under Tony Blair when he was prime minister.
His plans were approved, but later overturned by David Cameron when he became prime minister in 2010.