LONDON — Prime Minister Margaret Thatcher's Conservatives were reeling Monday after British voters, selecting delegates to the European Parliament in Strasbourg, France, gave the party its lowest share of a national ballot this century.
As the final returns were announced, Thatcher aides tried to dismiss the convincing victory by the rival Labor Party as a routine protest vote midway through the prime minister's third successive term of office.
Independent analysts acknowledged that the practical effect of the Tory defeat may be minimal because of the Strasbourg body's limited powers. But they said the psychological impact could be crucial, strongly confirming a buildup of Labor momentum first suggested in public opinion polls earlier this year.
Thatcher avoided the press during a normal working day Monday, but she termed the election results "disappointing" in a handwritten note to the Conservative Party chairman, Peter Brooke. Brooke conceded that he "may well have made mistakes" during a heavily criticized election campaign and offered to resign.
An ebullient Neil Kinnock, head of the Laborites, burst into song in his native Welsh language at an early morning champagne victory celebration. Labor had promoted the European elections as a referendum on Thatcher's policies.
Later, Kinnock dismissed the possibility that the prime minister can still turn the tide with a widely anticipated reshuffling of her Cabinet.
"They haven't got many aces to put in place of the jokers," Kinnock said. Referring to Thatcher, he added: "The one card in the pack that won't be reshuffled is the queen. And it is the queen that is their most basic weakness."
Kinnock charged Thatcher's government with "arrogance" and "superficiality" and said of the prime minister's anti-socialist legislative program: "She has gone much too far."
If the vote for delegates to the European Parliament were repeated in a British general election, according to an analysis by British Broadcasting Corp. television, Labor would emerge with a 26-seat parliamentary majority. That compares with a 100-seat majority that the Conservatives currently hold in the 650-seat House of Commons.
Britons went to the polls to elect their representatives in the European Parliament last Thursday. But the last returns were not announced until late Monday.
The final count showed that Labor won more than 40% of the vote, up nearly 4% from its showing in the last European elections in 1984. Thatcher's Conservatives, meanwhile, got slightly more than 34% of the votes cast, down sharply from almost 41% five years ago.
The results mean that Labor will send 45 of Britain's 81 representatives to Strasbourg, compared to the Conservatives' 32. In 1984, the figures were just the reverse.
The remaining four British seats went to small nationalist parties in Scotland and Northern Ireland. In all, there are 518 members of the Strasbourg assembly elected from 12 West European nations.
Because of its limited powers, the European Parliament has been described by some as a toothless forum for inflated oratory or, at best, the conscience of the Continent. However, it is playing an increasingly visible role as Europe tries to tear down barriers to free movement of goods and people by 1992.
Thatcher has been an outspoken critic of those who want to see the European Community evolve into a political entity more akin to a "United States of Europe." She fears that leftists in the European Parliament might push through legislation that could undermine the anti-socialist political revolution she has engineered in Britain since first taking office in 1979.
At the least, Labor activists hope to use their newly won clout in Strasbourg to put political pressure on Thatcher. One newly elected delegate gleefully predicted that he and his Labor colleagues at the European Parliament will make Thatcher "tear her hair out" in frustration.
The government's secretary of state for Scotland, Malcolm Rifkind, conceded in a television interview here that he was "deeply disappointed" after Scottish Conservatives lost the two Strasbourg seats they previously held. However, Rifkind stressed that the party has come back from mid-term defeats in the past to win "thumping" victories in general elections--"and I have no doubt the same will happen this time."
The government is not required to call a new general election until 1992, giving it as much as three years to turn the political situation around. Another ameliorating factor in the Strasbourg results was the unusually low voter turnout, 36%.
Still, Rifkind's Cabinet colleague, Energy Secretary Cecil Parkinson, took a more cautious line.
"We've not done well," he said. "This is a warning to the government. . . . We have got to get inflation under control. We have got to stick together. We have got to make sure the electorate understands the consequences of electing a Labor government."