Northern Ireland's two main Protestant political parties vowed today to end all political cooperation with British rule here to protest a new Anglo- Irish agreement for governing the troubled province, and said they would begin next week with the unprecedented mass resignation of their 15 seats in the Westminster Parliament.

Setting the stage for an all-out battle of wills, Democratic Unionist Party leader Ian Paisley and James Molyneaux of the Official Ulster Unionist Party said they intend to make Northern Ireland "ungovernable" by Britain. Any of their members who did not withdraw from British-run boards and official institutions in the province, they said, would be considered "collaborators and quislings."

Northern Ireland has been under direct British control in most matters since 1972, and the last major power-sharing initiative collapsed under Protestant pressure in 1974.

In addition to its own members of Parliament, Ulster's politicians and bureaucrats -- the vast majority from Paisley's and Molyneaux's parties -- participate in numerous advisory and regulatory agencies that run daily affairs here, and serve on elected county councils.

In a return volley to the Unionists, Northern Ireland Secretary Tom King, the British Cabinet minister in charge of the province, said attitudes like Paisley's and Molyneaux's were based on "ignorance and old tradition."

Regardless of their opinion of the agreement, which gives the Dublin government a role in the running of the north for the first time since partition of the island in 1921, he said, British Prime Minister Margaret Thatcher "is a very determined lady" and would refuse to hold a referendum on the accord in the province as demanded by the majority Protestant community, in the certainty that it would be rejected.

Instead, by resigning their seats in Parliament, the Protestant Unionist parties will force by-elections here. They would consider their reelection, they said, as proof of popular rejection of the accord.

The strategy is a risky one, since the central government in London is empowered to set the date for by-elections, and it could choose to delay them and deprive the Unionists of a forum. At the same time, the Unionists risk losing several seats now held with marginal majorities over Roman Catholic competitors.

King said Thatcher would not be intimidated by threats. "We are a sovereign government," he said at a news conference today. "We are determined to bring this agreement into effect."

In much of Belfast today, there was little overt enthusiasm for the accord, signed yesterday between Thatcher and her Irish counterpart, Garret FitzGerald.

A number of residents asked at random reflected what seemed to be two widespread views. Either the agreement would achieve nothing and would fail, as have all previous attempts to bring peace and stability here, or it would make matters worse and increase sectarian violence.

There were a few tentative good words for the historic accord, which gives Dublin a formal say in the province on behalf of the minority Catholic community through a joint Anglo-Irish ministerial council.

In a lengthy speech to a special session of the provincial Northern Ireland Assembly, Paul Maguire of the small Alliance Party, which advocates more reconciliation between Protestants and Catholics, urged everyone to read the agreement carefully before judging it.

Despite what others might say, Maguire saw it as no lessening of British sovereignty. "It gives Ireland a consultative voice," he said, "that would not be necessary in a normal society." But Northern Ireland is not a normal society. It is a society without political consensus, beset by terror, deeply divided, and administered . . . as a colony."

By the time Maguire spoke, however, the house was nearly empty. Most assembly members -- all of whom are Protestant, since the main Catholic-based Social Democratic and Labor Party long ago decided to boycott the body -- had left after Paisley's militant speech.

Implicit in the accord is the belief, not shared by many Protestants here, that the minority Catholics have been discriminated against within the Northern Ireland economy and political institutions, and by the courts and criminal justice system. As a result, they have become alienated and turned increasingly to the violent tactics of the outlawed Irish Republican Army, which aims to oust the British from the province.

The fight that began in earnest today between Britain and the Unionists involves longstanding protagonists; the Protestants have won all previous rounds on the issue.

When London tried in 1912 to introduce home rule over the Irish island that was its colony for centuries, the predominantly Protestant northeast fought it, arguing that its way of life would be overwhelmed by the Roman Catholic Irish majority. When Irish independence finally came 10 years later, the six northeastern colonies known collectively as Ulster were carved out, to the dismay of many Britons, to join the United Kingdom.

Violence between the Protestants and Catholics, who had remained as a minority in the new province, broke out periodically, flaring in earnest in 1969. In 1974, the British tried to alleviate it by orchestrating, along with Dublin and some Ulster leaders, a power-sharing agreement between the two communities. The Protestants, suspecting another attempt to sell them out to Dublin, protested again and won again.