The Thatcher government announced today its controversial decision to buy from the United States the Trident II submarine-launched, long-range nuclear missile system to modernize Britain's independent nuclear deterrent beginning in the 1990s.

After the decision was formally approved by Prime Minister Margaret Thatcher's Cabinet this morning, Defense Secretary John Nott told Parliament that the Reagan administration was enabling Britain to buy the system on "advantageous" terms for about $13.5 billion. He said this would consume less than a billion dollars a year of Britain's annual defense budget of more than $25 billion.

But critics of Trident here, including most opposition political leaders and some defense experts, contend that the cost will be much greater and will take money away from Britain's steadily shrinking conventional defenses.

With the bulk of the spending for Trident not scheduled to begin until after the next national election in 1983 or 1984, they have urged that it be canceled if there is a change in government.

Opposition defense spokesman John Silkin told Parliament his Labor Party "will cancel the Trident project" if Labor replaces Thatcher's Conservatives.

Liberal Party leader David Steel also indicated that the electoral alliance of the Liberals and new Social Democratic Party would do the same thing if it gained power.

Labor's deputy leader, Denis Healey, a former defense minister considered the party's staunchest supporter of the NATO alliance, told a group of American reporters today that Trident II, also known as D5, provides "far more nuclear capacity than we need and costs so much more that it takes money from our other defense needs."

"Our independent nuclear deterrent has been worth a good deal to Britain," Healey added, noting that its present Polaris submarine-based system will be in service another 15 years.

"But the time is coming when it would just be too expensive."

Similar controversy followed Thatcher's decision in 1980 to buy the smaller, less sophisticated Trident I (also called C4) system to replace Polaris. After President Reagan decided last year to switch from Trident I to II for the U.S. Navy, Nott and Thatcher decided to go along despite the greater cost of Trident II's more advanced technology and larger missiles and submarines.

The Trident II system provides 14 warheads on each of the 16 missiles per submarine, while Trident I missiles carry only eight warheads each.

Under an agreement negotiated by Nott and U.S. Defense Secretary Caspar Weinberger, Britain will build the four nuclear-powered submarines and the nuclear warheads it needs for the U.S.-made Trident II missiles. Weinberger also agreed to permit British defense contractors to compete on the same terms as U.S. firms for subcontracts to build components for both the British and U.S. Trident systems.

In addition, Weinberger agreed to a fixed research and development fee and waived other charges in exchange for British manning of Rapier air defenses around U.S. Air Force bases in Britain.

"The U.S. government is selling Trident D5 to us on more advantageous terms than Trident C4," Nott told Parliament, which must approve the Trident deal, "terms that protect us completely from development cost escalation."

Nott told Parliament that the Thatcher government "remains convinced that only Trident will provide a credible nuclear deterrent into the year 2000 and beyond. No other use of our resources could possibly contribute as much to our security and the deterrent strength of NATO as a whole."

Responding to critics who argue that Britain no longer can afford an independent nuclear deterrent, Nott said that "to choose a system lacking in credibility to an aggressor, or still more to abandon unilaterally a capability we have now maintained for three decades, would be a futile gesture that would serve to increase rather than diminish the risk of war."