Illegal aliens have become so numerous that those apprehended annually are almost double the number of foreign citizens entering the United States leglly, a Cabinet-level presidential committee reported yesterday.

Snowballing illegal immigration already has affected the labor market causing competition between aliens and lower income Americans for unskilled jobs, the committee's report said.

It added that strains on health and welfare services are likely to develop, particularly in big cities, as the aliens become settled and begin families.

Despite these problems, the report concluded, "massive deportation of illegal aliens is both inhumane and impractical." It recommended that U.S. policy should continue to stress a "preventive" course of discouraging and blocking the entry of illegal aliens, while allowing some of those already here to legalize their status.

What will be required ultimately, te report warned, "is a through rethinking of immigration policy in the contest over employment, population and foreign relations issues."

These are the principal findings and reommendations in the report forwarded to President Ford by the Domestic Council Committee on Illegal Aliens. Justice Department officials described it as "the broadest government assessment of the issue to date."

The committee, headed by Attorney General Edward H. Levi, included among its members the Secretaries of Labor, Agriculture, Commerce, State, Treasury, and Health, Education and Welfare, and the directors of the Office of Management and Budget and the Domestic Council.

Their report, which followed a year of study, cautioned that the true dimensions of the problem are not known because "hard data on illegal aliens is virtually nonexistent." For that reason, the report does not attempt to estimate how many people are in the country illegally.

The Immigration and Naturalization Service uses an estimate of 6 million to 8 million illegal aliens. But tha figure has been challanged by many who charge that ut us based on inadequate statistics and too much guesswork.

However, Immigration Service figures show that during the 1975 fiscal year 679,252 persons were arrested and deported as illegal aliens. During the same period, 386,194 legal immigrants entered the country.

The report notes that the greatest numbers of illegal aliens continue to come from Mexico and are concentrated in the Southwest near the almost 2,000 miles of U.S. Mexican border.

But, the report continues, the problem is by no means limited to Mexico. it notes that, since 1965 when the imigration law warged to loosen the national-origins quatas formerly weighted in favor of Europe, the source of both legal and illegal immigrants has shifted to Latin American and Asia.

The principal sources now, the report say, are Mexico, Jamaica, the Dominican Republic, Haiti, Korea, Thailand, the Phillipines and China.

All are developing countries with high rates of population increase and inadequate opportunities for the new people constantly entering the labor force.

"These push forces combine with the pull of available jobs and low risk of detection in the United States to produce illegal migration," the report says. It adds: "This pattern will apply to increasing numbers of countries in the future."

In discussing the impact on the labor market, the report cites the "bracero" program that formerly permited groups of Mexicans to work in the United States for limited periods. The progrm was ended in 1965 as the result of pressure from U.S. labor unions.

"Termination of the bracero program showed that sudden removal of alien labor opened jobs which natives took at improved wages, while income to owners decreased the consumer prices for their products increased," the report says.

The report notes the attraction for many employers to hire illegal aliens "who complete effectively with native workers, particularly with the minimally skilled and the underemployed."

"The immigrants raise the income of owners of capital and land and of highly skilled workers and lower prices to consumers of goods and services they help produce.Thus certain legal residents gain and others, particularly those with few skills, lose from the presence of illegal aliens."

However, the report continues, "the unskilled labor which the illegal (alien) contributes in the early stages of migration later tends to be offset when new communities of amilies must be absorbed . . . The migrant's aspirations, objectives and opportunities become attached to the host country so that he remains, establishing or sending for family rather than returning."

The result, the report adds, is to "create communities concentrated in our largest urban areas whose existence depends on avoidance of law and authority . . . The ramifications of harboring large numbers of people in illegal status are undersirable and contribute to a breakdown in the institutions and systems upon which we depend for fair govenment."

For these reasons, the reports suggests consideration of legislation that would enable some illegal aliens, already established in the United States, to legalize their status.

Other legislative reforms, the report adds, should aim at inducing some illegal aliens to leave by making their opportunities here less attractive. One method suggested by the report is stiff penalties for employers who knowingly hire illegal immigrants.

But, the respect concludes. "It is vastly more desirable from both a policy and resources standpoint to prevent entry of the illegal before arrival than to locate and apprehend the illegal once he is in the United States."

But to do this successfully, the reports says, it will be necessary to provide the Immigration Service and the State Department with greater resources and to establish better cooperation internally between federal agencies with immigration responsibilities and externally with foreign governments. The report does not specify how many more people or how much more money should be added.