LONDON — THE surprise defeat of the Conservative candidate in a by-election last Thursday has dealt a severe blow to Prime Minister Margaret Thatcher's apparent plans for early elections. The loss of the supposedly safe parliamentary seat of Eastbourne in the south of England appears to signify that, despite Mrs. Thatcher's claims to the contrary, British voters are taking a harsh view of the government's handling of the economy. It is also likely to lead to questions about the effectiveness of Thatcher's leadership after 11 years in office.
``Clearly the result is very disappointing and a very poor one for us,'' said Kenneth Baker, Conservative Party chairman.
The Eastbourne seat had been occupied by Ian Gow, one of the prime minister's closest advisers. He was killed on July 30 in an Irish Republican Army car bomb attack. Conservative strategists had counted on a heavy sympathy vote; instead, voters turned a 17,000-vote Conservative majority into a 4,500-vote margin for the centrist Liberal Democrats.
The opposition Labour Party candidate came a poor third in a district that has never been favorable to Labour.
Thatcher and her advisers had hoped that decisions earlier in the month to take the pound sterling into the exchange-rate mechanism of the European monetary system and to lower interest rates to 14 percent would be seen as signs of firm leadership. But voters appeared to be more impressed by inflation currently running at close to 11 percent and other indications of economic difficulty.
Mr. Baker said the anti-Conservative vote was a protest against the government's tough antiinflation policies. By next year, when inflation would be coming down, voters would think differently, he said.
The Tories had hoped to rekindle their declining fortunes with an early election next year. But Baker said yesterday that it was too early to talk about dates for a general election. Many Conservative members of Parliament are said to rule out a mid-1991 poll and to believe that Thatcher may find it difficult to go to the people with any confidence before 1992.
The latest date Thatcher can call a general election is mid-1992, but to gain the advantage of surprise it is usual for prime ministers to go to the voters well before Parliament runs its full five-year course.
Political analyst Ivor Crewe noted that by-elections are not always good guides as to how voters would behave at a general election. ``It is possible that Eastbourne is a one-off result. On the other hand, this is a major upset for the Conservatives, and it has to be bad news for Mrs. Thatcher,'' he said.
It is good news, however, for the Liberal Democrats and their leader, Paddy Ashdown, an ex-Royal Marine. For the past two years, the party's fortunes have ebbed as opposition voters appeared to be moving back to Labour. Mr. Ashdown has insisted that his party is not the ``dead parrot'' that Thatcher and the Labour leader, Neil Kinnock, have claimed it is.
A Labour strategist said there was a danger that if the Liberal Democrats were able to stage a significant revival, opposition votes at the general election would be split between Labour and the Liberal Democrats, giving the Conservatives a commanding edge.