"I don't believe that sacrifice of due process can be justified," said Major, Conservative premier from 1990 to 1997.
Gordon Brown wants to extend the maximum time limit that terrorism suspects can be held without charge to 42 from 28 days, but opposition parties and Labour rebels oppose the move as an infringement of civil liberties.
Defeat in a parliamentary vote on the proposal next Wednesday would severely wound Brown.
Major, whose cabinet was the target in 1991 of a rocket attack by Irish Republican Army guerrillas, argued that Brown's plans pose a graver threat to liberty than terrorism.
"If we are seen to defend our own values in a manner that does violence to them, then we run the risk of losing those values," he wrote in The Times.
"Even worse, if our own standards fall, it will serve to recruit terrorists more effectively than their own propaganda could ever hope to," he added.
Hoping to win over sceptics, Home Secretary Jacqui Smith this week submitted amendments to the proposed law, under which the 42-day detention powers would only be used against a "grave exceptional" threat - envisaged as something on the scale of suicide bombings in London that killed 52 commuters in 2005.
She reduced the time before which parliament must approve any use of the powers to a week from a month, and said the police would only have access to the powers for a limited time.
But that was not enough to placate critics.
Brown could still face a rebellion by up to 50 Labour MPs.
In a boost to the Labour rebels, parliament's joint committee on human rights said on Thursday the proposed amendments to the bill were negligible and that the move would still violate the European Convention on Human Rights.
But Brown did win support from some relatives of bomb attack victims polled by the Sun tabloid newspaper.
John Taylor, whose daughter Carrie died in the 2005 London attacks, said: "We believe that the 42 days is necessary to fight terrorism."
