LONDON, OCT. 19 -- Prime Minister Margaret Thatcher's Conservative Party suffered a political setback today, losing a House of Commons seat in one of its traditional strongholds to the minority Liberal Democrats in a special election.

The reversal in a southern England seat held continuously by Conservatives since 1906 surprised party leaders and could mean a delay in the national election Thatcher had hoped to call sometime next year. It was also considered a personal blow to Thatcher because the seat had been held previously by Ian Gow, a close friend and political soul mate who was killed by the Irish Republican Army in a car-bomb attack in July.

David Bellotti, the Liberal Democrat candidate, won 51 percent of the vote to 41 percent for Richard Hickmet, the Conservative in Thursday's balloting, the results of which were announced early this morning. The candidate of the opposition Labor Party took just over 5 percent, barely enough to win a refund of the bond all candidates are required to post when declaring for office.

It was a swing of 20 percent against the Conservatives, who captured the seat with 60 percent of the vote just three years ago.

Analysts said that Labor's poor showing was partly due to a last-minute change in candidate and to the decision of many traditional Labor voters to switch over to the more middle-of-the-road Liberal Democrat when they saw he had a chance to win.

British parliamentary special elections are notoriously fickle affairs because they allow voters to send a protest message to the ruling party without actually voting it out of office. The Conservatives, who hold a 100-seat majority in the 650-member House of Commons, could take some solace from an exit poll indicating that had this been a regular election contest, their candidate would have won.

Nonetheless, politicians and analysts agreed that the loss of the Eastbourne seat was an important setback for Thatcher and that it indicated deep public distress with the government's handling of the economy, its plans to reform the National Health Service and its imposition of a new system of local taxation known as the poll tax.

Conservative Party Chairman Kenneth Baker said the result was "disappointing" but called it a "protest vote against the very tough policies that the government has been correctly following in order to bring inflation down." Britain's annual inflation rate is nearly 11 percent and interest rates remain at 14 percent.

Thatcher last week had mocked as "a dead parrot" the Liberal Democrats, who are seeking to emerge as a third force from the ashes of the defunct alliance of the old Liberal and Social Democratic parties. "The parrot has twitched," Baker conceded today.

Paddy Ashdown, leader of the Liberal Democrats, said the vote marked a new era in British politics. "The end of Thatcherism has begun," said Ashdown, who claimed the result would prove damaging not only to the Conservatives but also to Labor, which has sought to eliminate third parties such as the Liberal Democrats.

Conservative candidate Hickmet was criticized for running a campaign that seemed to overplay sympathy for Gow's killing and suggested that a vote for another candidate would be an endorsement of the Irish Republican Army. "Any result, other than a massive vote for Richard Hickmet, will be a moral victory for terrorism," read one leaflet.

Many Conservatives had wanted Thatcher to call the next election next summer or fall when, they hope, interest rates and inflation will be falling in part because of Britain's recent decision to join the European exchange rate mechanism. She is not legally required to call an election until the summer of 1992.