Prime Minister Margaret Thatcher's government could not have foreseen and probably could not have prevented Argentina's invasion of the Falklands last April 2 even if every hint in advance had been correctly assessed, an official investigation into the causes of the war declared today.

"We conclude," a blue-ribbon panel reported after a six-month inquiry, "that we would not be justified in attaching any criticism or blame to the present government for the Argentine junta's decision to commit its act of unprovoked aggression."

That exoneration of the Thatcher government--which the prime minister pointedly read out to Parliament this afternoon in presenting the eagerly awaited report--is the main political finding of a study based on complete access to intelligence assessments and official deliberations dating back to 1965. So sweeping is the conclusion that Thatcher's critics will be hard-pressed to score points against her based on what the investigation found.

Under the chairmanship of Lord Franks, a distinguished scholar and former ambassador to Washington, the panel asserts in its 106-page report that the invasion decision clearly was taken by the junta "at a very late date" and that the government could not, therefore, have had "earlier warning." Moreover, the report says, "there is no reasonable basis for any suggestion" that a different British policy as the crisis developed would have forestalled the Argentine action.

However, the report does note some shortcomings, particularly in the handling of intelligence. It notes a number of points in the months leading up to the war where "different decisions might have been taken, where fuller consideration of alternative courses of action . . . might have been advantageous and where the machinery of government could gave been better used."

That these did not occur, the study suggests, was the fault, at least in part, of an inadequate assessment of Argentine intentions by the Joint Intelligence Organization. This is a coordinating group made up of officials from several government departments, under the leadership of the Foreign Office--which the report, in turn, also criticizes for underestimating certain signals from Buenos Aires.

If there is any direct consequence of today's report, it is likely to be a shake-up in the intelligence committee to give it greater independence from diplomats. The study says the group failed to credit adequately the increasing militancy of the Argentines in early 1982, both in the press and through diplomatic exchanges. It said the committee relied on "secret intelligence which at that time was reassuring about the prospects of an early move to confrontation."

In addition, the study said, the impact of cumulative British actions, which might have given Argentina the idea that the Thatcher government would not react decisively to an invasion, was underestimated by the committee in predicting Argentine plans.

These included the decision, taken twice by successive governments, to withdraw the Falklands patrol ship of the Royal Navy, HMS Endurance, to save money; plans to withhold full British citizenship from some of the 1,800 Falkland Islands residents, and the failure of two British governments to implement a 1976 report on developing the islands.

The study singles out no individuals for criticism. But neither does it question the resignation of foreign secretary Lord Carrington, who called the invasion a "national humiliation." Carrington was in Israel on March 31, the day British intelligence firmly reported that an invasion could take place on April 2.

In the months preceding, the panel writes, the Foreign Office "did not attach sufficient weight . . . to the changing Argentine attitude . . . and did not give sufficient importance to the new and threatening elements in the Argentine government's position."

The Franks panel was created in July, a month after a British task force successfully reclaimed the Falklands. It had six members--two from the Conservative Party, two from Labor, Franks, who is a member of the Liberal Party, and a retired senior civil servant.

Opposition politicians clearly hoped to turn Thatcher's gains in popularity after the conflict into liabilities by pinning her and the government with the blame for not avoiding it. Despite the report's conclusions absolving Thatcher, Labor Party leader Michael Foot told Parliament that it showed "a complete collapse of effective Cabinet government," an apparent reference to Thatcher's dominance of decision-making and perhaps to the problems that were revealed in coordination of intelligence among ministries.

Thatcher, doubtless pleased and relieved at the panel's findings, praised it for producing a "thorough and comprehensive report in so short a time." Thatcher returned from a triumphal tour of the Falklands last week.

The U.S. role in advance of the war, as portrayed in the study, appears to have been small and there is no indication that American intelligence information played any part in Britain's underestimate of the Argentine plans.

Then-secretary of state Alexander M. Haig Jr. was informed by the British of "Argentine intentions" April 1. Interviewed by BBC television tonight, Haig said the United States had not been looking for and did not seek indications of the invasion in the weeks before it happened. On learning from the British that war was imminent, the study said, President Reagan then attempted to persuade the junta leader, Gen. Leopoldo Galtieri, by telephone not to invade. But he was unsuccessful.