After six years of wrenching national debate, the Dutch government announced its willingness today to deploy 48 cruise missiles but said it will abandon two other nuclear tasks carried out for the NATO alliance.

Prime Minister Ruud Lubbers, emerging from a 12-hour Cabinet meeting, said his center-right ruling coalition would seek to pass final legislation bringing the missiles into the country in 1988 because the Soviet Union had expanded its force of SS20 missiles beyond the level deployed in June 1984.

The Dutch government declared at that time it would refuse its share of cruises under NATO nuclear modernization plans if the Soviet Union did not add to its arsenal of 378 SS20s. But the latest NATO figures show that 441 of the triple-warhead SS20s are now operational in the Soviet Union.

In opting to accept the missiles, Lubbers overcame considerable opposition within the country and his own Cabinet and rebuffed intense eleventh-hour approaches by Soviet leader Mikhail Gorbachev. Some ministers had argued for a further delay because of new arms control proposals advanced by Moscow and Washington and the imminence of the Geneva summit meeting Nov. 19 and 20 between President Reagan and Gorbachev.

The decision came as the United States presented the Soviet Union with a new arms proposal in Geneva today and the Soviets agreed to extend the talks to consider the plan.

In Washington, State Department spokesman Joseph W. Reap Jr. called the decision "a welcome step" demonstrating continued Dutch "adherence to the fundamental principles underlying the (NATO) alliance."

In a letter to the speaker of the Dutch parliament that was also signed by the government's foreign and defense ministers, Lubbers said that "further deferment would undermine the credibility of the Netherlands' policy and call into question its reliablity as a NATO partner."

The prime minister also noted that putting off a decision would betray signs of weakness and division within the western alliance and tempt Moscow to seek to gain a unilateral cut in nuclear missiles by the West without an arms control pact.

An additional factor, Lubbers said, is that Nov. 1 is considered the last possible date to allow construction of the missile site to proceed so the NATO deployment schedule could be met by 1988.

The government also released a series of letters exchanged by Lubbers and Gorbachev in recent weeks as the Soviet Union intensified its last-minute appeals to the Dutch government to reject the missiles.

On Oct. 2, Gorbachev wrote Lubbers to emphasize that some SS20s had been removed from standby alert and would soon be dismantled so the same number of 243 SS20s would be targeted on Western Europe as in June 1984. But Lubbers insisted his government would count all SS20s in both eastern and western parts of the Soviet Union because it is a mobile missile.

Three weeks later, Gorbachev wrote again to stress that new Soviet proposals in Geneva offering a 50 percent cut in strategic nuclear weapons had improved negotiating prospects. He said the Netherlands should also make its contribution to improving chances for arms control and better East-West relations by declining the cruise missiles.

Unlike June 1984, Gorbachev continued in the letter, the two superpowers were now negotiating serious arms cuts and everything possible should be done to enhance prospects for an agreement.

Last Wednesday, Anatoli Blatov, the Soviet ambassador to The Hague, delivered a last-minute request urging a suspension of the cruise decision and offering direct talks with Moscow.

But Lubbers said the Soviet Union must bear responsibility for not taking the necessary steps during the past 17 months to comply with the condition stipulated by his government for a rejection of cruise missiles.

The prime minister said today he was still prepared to travel to Moscow to discuss nuclear weapons as long as the Soviet invitation did not attach unacceptable terms such as renunciation of the missiles or delays in deployment.

The decision to accept the missiles had been delayed twice in recent years because of staunch opposition organized by church groups, left-wing political parties, trade unions and peace activists in the Netherlands.

But the antimissile crusade has weakened substantially within the past year as other Western European allies successfully deployed missiles with little unrest and a sense of exhaustion began to afflict missile opponents after years of fruitless demonstrations.

The Dutch government also sought to insure a less critical response to today's decision by renouncing other nuclear duties the Netherlands has performed within NATO.

Lubbers said his government would forgo all nuclear missions carried out by F16 fighters and the Orion reconnaissance plane, which is capable of dropping nuclear charges. Nuclear land mines and the Nike-Hercules antiaircraft system are also being withdrawn because they have become obsolete, leaving only the Lance guided rockets and short-range artillery as the country's remaining nuclear functions in NATO.

The government now plans to push through legislation so construction can begin early next year at the Woensdrecht missile base in the southern part of the country as well as finalizing the treaty with the United States over control of launching orders.

The opposition Labor Party hopes to stall final passage in the coming months so that the controversy might linger as a campaign issue prior to the next national election in May.

Current opinion polls show that the governing coalition of Christian Democrats and Liberals would lose its majority if elections were held now. But Lubbers and his partners expect that defusing the missile crisis and encouraging a gradual economic recovery will improve their chances of staying in power.