Thatcher Determined to Carry On Despite By-Election Setback : Britain: Labor’s win of a ‘safe’ Tory seat may reflect a tide against her. But she rejects ‘trimming and turning.’

A “disappointed” Prime Minister Margaret Thatcher said Friday that despite one of her Conservative Party’s most humiliating by-election defeats in more than half a century, she will stick to the policies that have kept her in office for nearly 11 years.

“We are not a ‘fair-weather’ party,” the 64-year-old prime minister stated in a letter to party Chairman Kenneth Baker. “We are not for trimming and turning.”

The Conservatives were drubbed at the polls Thursday in a semi-rural constituency north of Birmingham where they had enjoyed a large majority at the last general election in 1987.

Final results announced early Friday showed the rival Labor Party candidate, social worker Sylvia Heal, taking 49% of the vote, against 33% for her Conservative competitor, publisher Charles Prior. The remaining votes were shared among other candidates representing smaller parties.

Heal’s 9,449-vote margin of victory marked a stunning turnaround. In 1987, John Heddle won the seat for the Conservatives with a 14,654-vote cushion. Heddle’s suicide last December triggered Thursday’s special election to fill his vacant seat.

The vote was widely seen here as evidence of a powerful anti-Thatcher tide among voters that seriously jeopardizes her chances of gaining a fourth term in office at the next general election, which must be held before mid-1992.

Labor Party leader Neil Kinnock said the voting had marked more of a turning point for the Conservative Party than for his own.

“They are going to have to make up their minds how quickly they get rid of Mrs. Thatcher,” he said.

In her letter to Baker, the prime minister acknowledged that the voters of the Mid-Staffordshire constituency had “taken their chance to send us a message” about high mortgage interest rates and an unpopular new local taxing scheme. “These matters are naturally of concern to us all,” she said.

However, Thatcher insisted, “I know that by continuing the policies which have served us so well, we will win back the support of the electors of Mid-Staffordshire at the next general election.”

A close Thatcher aide said the prime minister considered the Mid-Staffordshire result to represent a protest rather than a sea-change in the national political mood.

Ruling parties here traditionally suffer a slump in the middle of their terms of office. They almost always recover somewhat before the next election, although sometimes not enough to stay in office.

According to the latest British Broadcasting Corp. compilation of the four most important polls here, Conservative support nationwide has fallen to 30% from 45% at the beginning of 1989. The party won the 1987 general election with 43.3% of the vote. Satisfaction with Thatcher’s leadership stands at an all-time low of 26%, the BBC said.

What most worries the Conservatives is that the erosion in support has come mostly among those people who have been the primary benefactors of Thatcherism: young homeowners and skilled workers. They are hit hard by one of Europe’s highest inflation rates and a government policy of keeping interest rates high that has pushed mortgage payments up by several hundred dollars.