Thatcher shuffles Cabinet in bid to boost fortunes. Shakeup seen as preparation for Britain's next general election

British Prime Minister Margaret Thatcher has geared up her government to fight in the 1987 general election by making key changes in her team. The aim of the prime minister's most extensive Cabinet shakeup in the six years since she took office, government officials said, was to ready her Conservative Party for a sustained effort to recoup the government's waning political support across the country.

At the cutting edge of the changes announced Monday was the sacking of John Selwyn Gummer, the party chairman, and his replacement by the ambitious and influential Norman Tebbit.

Mr. Tebbit, who has been secretary for trade and industry, is expected to take firm control of the Tory party machine. The party has not prospered under Mr. Gummer, a relatively junior figure, who, according to sources here, lacked authority.

Mrs. Thatcher also surprised the political community by moving Home Secretary Leon Brittan into Tebbit's old job. Mr. Brittan urged the banning of a British Broadcasting Corporation television film last month about Northern Ireland.

Brittan was said by government insiders to have displeased the prime minister with his handling of the subsequent flap. He lacks a power base in the Tory party, and has been effectively demoted, political sources said.

But at 10 Downing Street, Brittan's transfer was characterized as part of an attempt to boost industry and tackle unemployment.

In a move thought to be unique in the runup to a cabinet reshuffle, Thatcher let it be known privately that she did not intend to bring back into her inner ministerial circle Norman Parkinson, the former party chairman who two years ago was forced to resign when it was revealed that he had been having an affair with his secretary.

Thatcher was angered by the forced removal of Mr. Parkinson, whom she sees as a personable and effective senior politician, and wanted him back. But senior Cabinet members warned her against creating more trouble, and -- in a serious setback to her personal authority -- she accepted their advice.

An indication of the prime minister's problems as she confronted the Conservative Party's poor performance in the opinion polls came from Tory Members of Parliament on the eve of the Cabinet reshuffle.

To get renewed success with the voters Thatcher and the government have to convince people that its qualities and successes are greater than its mistakes and failures, Member of Parliament Kenneth Lewis wrote to the Times (London).

Even a year ago such talk would have been unthinkable from a senior Tory party figure. Thatcher is under mounting pressure to provide the kind of leadership that made her an overwhelming victor in the 1983 general election.

In appointing Tebbit as party chairman, she may be taking a gamble with her own future. A tough-talking former airline pilot with a working-class background, Tebbit is known to have the ambition to lead the Tories.