The red British postal trucks here are labeled "Royal Mail" on one side and "Post Brenhinol" on the other. Signposts pointing the way to this city, the largest in Wales and its administrative capital, call it both "Cardiff" and "Caerdyff." Street names are posted in Welsh and English. Welsh-language television programs, including familiary American movies dubbed in Welsh, are broadcast on all channels.

These are the outward signs of the remarkable progress made in the past decade by a small Welsh-speaking minority -- less than 500,000 of the 2.7 million residents of Wales -- in reviving the Welsh language and Welsh nationalism after seven centuries of British rule.

The years of protest demonstrations, courtroom sit-ins, bombings of television transmitters and destruction of English-only signs have won many gains. All highway and street signs, income tax forms and other government documents throughout Wales are now bilingual.

Welsh is taught and used as the language of instruction for other subjects in hundreds of schools. Some local government meetings and courtroom proceedings are conducted in Welsh. Fluency in Welsh is required for some jobs.

Three members of the Welsh nationalist party, Plaid Cymru (Party of Wales), have won seats in the British Parliament. The British government has given Wales its own London-controlled regional bureaucracy, the Welsh Office, and pumped extra economic development and social welfare money into Wales, where the coal and steel industries are dying.

But the Welsh nationalist movement, which originated in the Welsh-speaking rural villages of northern and western Wales, has run into an angry backlash from the English-speaking majority concentrated here in southeastern Wales, with its heavily populated cities and industrial valleys near the English border.

In a referendum election Thursday, the voters of Wales will decide whether they want the first limited form of home rule offered since Wales was conquered by the English in 1282 and formally incorporated in 1536.

Opinion polls show that those who intend to vote "no" outnumber likely "yes" votes by a wide margin, and leaders of the "yes" vote campaign here admit they are fighting an uphill battle.

It appears that the "no" voters are not so much against the proposal for a rather weak elected assembly in Wales as they are determined to stop the advance of the Welsh-speaking minority that they see behind the assembly proposal. Because most of the changes won up to now by the Welsh nationalists have been administrative, this is the first opportunity all of Wales has had to vote on the issue of nationalism and the role of the Welsh language.

"We didn't ask for bilingual signs and tax forms or for Welsh television programs," said Alex McKay, an English-speaking worker campaigning against the referendum. "We wonder if Welsh will be required for the civil service if the assembly is approved."

Supporters of the assembly argue that it will give the voters of Wales control over the bureaucrats who now run Welsh affairs under administrators appointed by the British government. The assembly would decide how to spend the money allocated for Wales by London and set policy for education, housing, employment, economic development and other regional concerns. It would not have the power to raise taxes or make laws.

It would be impossible, argue assembly supporters, for the Welsh-speaking minority, which constitutes less than 20 percent of the population, to use the assembly to shove the Welsh language or nationalism down the throats of the rest of the English-speaking majority.

But editor Geoffery Rich of the South Wales Echo here said he is convinced that the Welsh nationalists will nevertheless try to use the assembly as a vehicle to press for further spread of the Welsh language and for ultimate independence from Britain.

"I'm for saving the Welsh language as long as it doesn't get in the way," said Rich, whose newspaper is campaigning strongly for a "no" vote. "I don't want my kids to have to learn Welsh in school, and I don't want people to have to learn it to get a job."

Plaid Cymru and the activist Welsh Language Society are quite open about their determination to achieve independence from Britain for a fully bilingual Wales. Their leaders have said they are supporting the assembly proposal, which they found very disappointing in its limitations, only as an interim step.

Plaid Cymru was founded in 1925 by a few writers and teachers in Wales to preserve the dying Welsh language. After one of the founders, Saunders Lewis, made an emotional speech in 1962 calling for action to save the language, the Welsh Language Society was formed by university students in the 1960s.

Their civil disobedience and Plaid Cymru's parliamentary election victories applied the pressure that resulted in official protection for the Welsh language by act of Parliament in 1967 and the subsequent spread of bilingu-alism in Wales by British government decrees.

Editor Rich argues that the millions of dollars spent on bilingual road signs, government forms and Welsh instruction in the schools could better be spent trying to solve the acute unemployment problem in southern Wales.

Like many people here, Rich talks of "South Wales" and "Welsh Wales" as though they were separate countries. Many residents of southern Wales are descended from coal miners and iron workers who immigrated from England and still have ties with nearby English cities. They see the Welsh speakers as a different people with a foreign language and customs.

Prime Minister James Callaghan, who represents a southern Wales constituency in Parliament, tried to quell their fears in a major "yes" campaign speech at a Labor Party rally in the nearby southern Wales city of Swansea. Loyal Labor voters in the mining and steel towns in the valleys of southern Wales hold the swing vote in the referendum.

"Those who peddle the nightmare of separatism show little faith in the Welsh people," Callaghan said, arguing that creation of the assembly would give Wales enough home rule to reduce the appeal of separatists.

"An assembly for Wales would strengthen the sense of Welsh nationhood," he said. "But by removing what has become, for a lot of people, a source of grievance, it will pull the rug out from under those who really do want to break our united island into separate pieces."

Callaghan's Labor government pushed the home rule proposal through Parliament, along with a companion measure that offers an assembly with more powers to voters in Scotland in a referendum on Thursday there. Both proposals were designed to prevent further nationalist inroads into traditional Labor strongholds in working class Wales and Scotland.

In the short run, Labor also is working hard for approval of the home rule plans to keep the support of the three Welsh nationalists and eleven Scottish nationalists in Parliament. Without them, Callaghan's government would not survive a vote of confidence and would have to call an immediate national election. He would like to avoid a national election until summer or fall, when he hopes Labor will be doing better in public opinion polls.