LONDON — BRITAIN has supplied a possibly decisive boost to the European Community's drive toward economic and monetary union. In a speech to members of his ruling Conservative Party, Prime Minister John Major called on them to stop quarreling over European policy and leave him free to negotiate the next stage of a European Monetary Union (EMU) treaty to be signed in December. His aides described the speech, given Friday, as the most important of his political career.
Mr. Major deliberately distanced himself from the Euro-skepticism of Margaret Thatcher, his predecessor as prime minister, and signaled he was prepared to delay calling a general election until the December treaty had been signed.
One effect of Major's stated determination that ``Britain must not be sidelined'' in the European debate is that the 12 EC members are now much more likely to remain united in their quest for economic and - eventually - political integration.
There are strong indications that Britain and Germany are cooperating closely on moves toward European integration.
Sources close to Mrs. Thatcher said yesterday that, in the wake of the Major speech, she would be ``modifying'' an address on European integration that she was due to give in Chicago today. Instead of restating her well-known opposition to a federal Europe, the sources said, she would be at pains to avoid an open clash of views with Major.
Major's decision to draw a firm line between his and Thatcher's approach to Europe was seen afterward by some senior Conservatives as the speech's most significant feature. In fact, however, it was much more than an attempt to cut loose from the woman who groomed him for the premiership.
The speech embodied a two-prong strategy: to unite the Conservative Party and to reassure the other EC partners that Britain was no longer Europe's ``odd man out.'' It would not try to veto changes to the Treaty of Rome (the EC's founding Constitution) needed to give the Community a common currency and a central bank.
The Major government is doing badly in the opinion polls. In the past week, two leading polls have given the Labour Party opposition a 10 point lead over the Conservatives.
Disunity within the Conservative Party, polling organizations say, is a key reason for its unpopularity.
Senior EC officials welcomed the Major speech which, they said, meant that the British prime minister had embraced a compromise for achieving EMU devised by Jacques Delors, president of the European Commission.
Under the Delors formula, Britain would sign a treaty on EMU at a summit meeting in Maastricht, the Netherlands, in December, but could reserve the right to delay acceptance of a European currency until a later date.
Sir Leon Brittan, a vice president of the European Commission and a former British Cabinet minister, welcomed Major's acceptance of the Delors compromise. He said Saturday it would ensure that moves toward EMU could proceed ``without undue delay.''
Sir Leon added: ``Under the plan, Britain will not have to take a decision to join in a single European currency until 1997 or 1998. There is plenty of time.''
Until last Friday, Major had hesitated to adopt the Delors approach. Meanwhile, an angry debate raged within the Conservative Party between pro-European members and the antifederalist Bruges Group, whose president is Thatcher. At one point, Thatcher appeared to have entered the dispute personally when criticisms of Major reportedly made by the former prime minister appeared in Conservative newspapers. She was said to be furious about his failure to condemn EC moves toward a single currency.
The debate reached a crescendo last Wednesday when Nicholas Ridley, a former senior government minister and outspoken Thatcherite, said he and Bruges Group members were prepared to split the Conservative Party rather than allow the government to sign up for EMU.
According to Major's senior colleagues, this was the last straw for the prime minister. He decided to turn a scheduled speech to Welsh Conservatives into a platform for denouncing the antifederalists and throwing his personal authority behind a bid to sign an EMU treaty, with safeguards, by the end of the year.
Government sources indicated that Major worked closely with Douglas Hurd, the foreign secretary, to develop a line of argument that would slam Conservative critics and simultaneously give a signal to the Brussels headquarters of the EC that Britain was serious about EMU.
The resulting address to a conference in the Welsh capital, Swansea, was a blend of commitment to the unity of the Conservative Party and the EC.
It made it plain that the British Parliament would make the final decision on acceptance of a single currency, but that in the meantime Britain would play a full part in negotiations on amending the Treaty of Rome.
Major was equally determined to reject the views of what he called Conservative ``faint hearts'' opposed to economic and monetary union.
Chris Patten, the party chairman, had warned Major that unless Conservatives closed ranks and stopped bickering about Europe and other matters, they stood to lose the general election that must be held by mid-1992.
In a key passage in his Swansea speech, Major declared: ``Sulking on the fringe of talks about the destiny of Europe can never be the right role for Britain. Europe is not a battle between `them' and `us.' It is our continent, too.''
In another part of the speech, the prime minister said: ``We must not - cannot - turn our backs on the construction of the new Europe. That is where our history points; and out interest lies.''
His officials later privately pointed to the difference in tone and approach on Europe between Major's statement and the line followed for nearly 12 years by Thatcher, who often spoke in ``them'' and ``us'' terms.