President Reagan is expected to tell Congress next week that he supports the 1986 federal pay freeze included in the recent Senate-House budget compromise, despite a new determination by administration officials that white-collar workers are due a 19 percent raise in October.

The president's pay agents -- the Secretary of Labor and the directors of the Office of Management and Budget and the Office of Personnel Management -- have concluded in their as-yet unreleased annual report that there is a wide gap between salaries paid for similar civil service and private industry work.

That data, based on a pay survey done this year by the Bureau of Labor Statistics, indicates that it would take a federal pay raise averaging 19.15 percent in October to close the gap.

But as they did last year, the pay agents are expected to recommend that the raises be set aside until the BLS can expand its survey to include more private firms and more jobs that are representative of the kinds of work done by the nation's 1.4 million civil servants.

The average salary in the United States last year -- including those of federal workers -- was $18,350, according to the Labor Department. Alaska and the District of Columbia had the highest average salaries -- $28,806 and $25,120, respectively. The lowest average salaries were in Mississippi ($14,398) and Maine ($14,850).

The average white-collar federal worker outside Washington earns $25,247, while the average federal salary in the Washington metropolitan area this year is $31,187, according to the Office of Personnel Management.

The average annual pay for 93 million workers takes into consideration low-paying jobs that are rare or nonexistent in the federal civil service.

And although the government does pay differentials to about 30,000 workers in high-cost areas, federal jobs in a given grade generally carry the same salary regardless of location.

The BLS says the pay differentials between government and industry result from the salaries paid to government attorneys, scientists, engineers and professional employes, which are lower than those of their counterparts outside of government.

Federal workers are supposed to get a raise each October, based on the BLS data, to allow their salaries to catch up with industry's. But federal law gives the president the authority to raise, lower or delay any increase by simply submitting an alternative plan to Congress before the first of September.

The president's alternative proposal goes into effect automatically unless the House and Senate pass separate pay legislation and the president approves the legislation, or unless Congress overrides a presidential veto.

In his budget in January, Reagan proposed a 5 percent federal pay cut for 1986. He could still get it. But congressional sources consider it unlikely, because the Senate and House have approved a pay freeze for next year.