Union negotiators for Britain's half million civil servants walked out of pay talks today, rejecting the government's offer of a 7 percent wage increase and vowing to bring the bureaucracy to a halt with a nationwide strike.

Selective strikes by the civil sevice unions during the past month already have shut down many government computers, interrupted cable communications between the British Foreign Office and its embassies around the world, and closed the courts in Scotland.

Today, a walkout by customs officers shut down air freight operations at London's busy Hearthrow Airport.

The civil servants, who earn $10,000 to $30,000 a year, want raises of 20 to 36 percent to bring their pay in line with what their unions say are salaries for comparable jobs in private business. Their leaders today called the government's 7 percent raise offer "wholly unacceptable."

The civil service crisis is the latest in a winter of labor strife that lingers on much as the dreary winter's cold and freezing rain have throughout most of the country.

Local government workers, whose strikes left trash uncollected and schools closed for months, went back to work after settling for raises averaging about 10 percent. Today, the union leaders of striking hospital workers, who forced hundreds of hospitals to limit admissions to emergencies, voted to accept a similar deal.

Now school teachers are threatening to strike in many parts of the country, as are provincial bus drivers. Today members of Parliament were deprived of their afternoon tea when the catering staff of the House of Commons' cafeterias and bars went on strike.

While the strikes actually have not seriously affected most citizens, the nagging persistence of the labor trouble has created an uneasy public mood that sent the Labor Party government tumbling in public opinion polls and helped undermine Prime Minister James Callaghan's efforts to avoid holding national elections until the labor situation settled down.

Now Callaghan faces a vote of no confidence in the House of Commons Wednesday that could force an election as soon as the end of April.

Margaret Thatcher, leader of the opposition Conservatives, served notice over the weekend that, whenever the election is held, the unions and the state of the economy will be prime issues in her campaign to become Britain's first woman prime minister.