Professional and service businesses in the District were warned by the city government last week that they must continue to pay a contested tax, pending a Supreme Court decision.

D.C. Finance and Revenue Director Carolyn Smith said her office has been flooded with inquiries from taxpayers who are uncertain about whether to file the unincorporated business franchise tax, which took effect at the start of 1975 but which has been successfully challenged to date in the courts.

Affected are about 4,500 self-employed professionals, including lawyers, accountants, architects, doctors and dentists as well as other personal service businesses. According to Smith, they are requireed to file their tax returns and pay the taxes due. The filing date was April 14 for businesses with a calendar year and the 15th day of the fourth month following the end of a fiscal year period for others.

At the same time, businesses involved in the dispute professional tax situation may file separate claims for refunds to get their money back if the Supreme Court upholds a lower court decision that the tax is illegal.

Questions about the tax should be directed the D.C. Department of Finance and Revenue, District Building, Washington 20004; the telephone number is 727-6094.