Western Europe decided unhesitatingly yesterday to extend indefinitely economic sanctions against Argentina, handing Britain a surprisingly easy diplomatic victory.

The action, which came early during a 16-hour session of European foreign ministers, appeared to be part of an effort by the European Community to demonstrate unity and firmness less than a week after it was plunged into its worst internal crisis in years.

Meeting into this morning at Brussels' Egmont Palace, the European ministers in turn drew a large enough monetary concession from Britain to reach a final compromise agreement on the vexing question of a 1982 budget rebate for Britain.

Sidestepping another controversy, the ministers elected to put off until next month a reassessment of the 10-nation community's voting principles. A 16-year-old tradition allowing member states to veto proposals of vital national interest to them was called into question last week when a shocked Britain watched its attempted veto over farm price increases shoved aside by seven community members.

Two of Britain's partners--Italy and Ireland--remained formally outside the ban on Argentine imports, having refused a week ago to say they would continue the embargo when the rest of the community agreed then to a week's extension.

But spokesmen for both Italy and Ireland--where overt solidarity with Britain in the Falklands conflict has become a domestic political risk--reported that their countries had in fact not begun receiving new imports from Argentina. Suggesting that quiet observance of the ban continued, the spokesman indicated that a resumption of Argentine imports probably was some time off because of administrative and legal delays or a lack of commercial interest.

The swift, open-ended extension of the five-week-old ban, which was due to expire last night, was welcomed by British Foreign Secretary Francis Pym, who thanked the other ministers for their support and solidarity at this crucial time for Britain.

At least two factors help to explain why the sanctions issue met with relatively little division this time. One is that the objections by Italy and Ireland already had been accepted and dealt with a week ago in a legal formula that seemed palatable to all community partners.

The second is that Britain, by publishing Thursday what its negotiating position on the Falklands had been, had made clear how much it was willing to concede before the negotiations with Argentina fell apart. This appeared to answer concerns among European Community members that backing of sanctions not reduce London's interest in pursuing a diplomatic solution to the South Atlantic conflict. It also seemed to reinforce Europe's judgment that Argentina was mostly to blame for the deadlock.

Some of Britain's allies still were anxious to stress today that they wanted to see a political rather than a military solution to the Falklands crisis. Dutch Foreign Minister Max van der Stoel told reporters that prolonging the sanctions did not mean a "blank check" for Britain.

"It is possible," he said, "to release the sanctions even within a month, assuming the Argentines agree to negotiate on the very reasonable U.K. proposals."

West German Foreign Minister Hans-Dietrich Genscher was quoted by a spokesman in Bonn as saying he hoped the military escalation in the Falklands "will not be continued further."

European ministers are expected to review the ban, which prohibits new import contracts with Argentina but exempts those existing before April 17, at their next scheduled meeting in June. The economic cost so far of the ban to the community reportedly has not been great.

A greater key to the community's future economic and political health is the nagging question of its budget. Under the terms of this morning's rebate compromise, Britain will get back $875 million this year from the community budget--which is roughly $400 million less than what London had originally asked for.

In return, Britain's community partners committed themselves to negotiating a longer-term solution to the British overpayments and other budgetary problems by November.

While senior European officials continued to deny any explicit link between the sanctions issue and the budget quarrel, it clearly has been the hope of Britain's partners that by showing solidarity with Britain in the Falklands conflict, they would encourage British Prime Minister Margaret Thatcher to make concessions in the budget dispute.

Preoccupation with the Falklands crisis in Britain seems to have provided the Thatcher government with an opportunity to make concessions with the minimum of domestic uproar.