The Reagan administration is considering a plan to merge two big grants to the states for services to the poor and cut the funding to $2.5 billion a year, nearly $300 million less than the current combined appropriations for the programs.

The proposal, which must be authorized by Congress and could meet considerable resistance there, is considered almost certain to be included in President Reagan's fiscal year 1984 budget proposals. It would unite the Social Services Block Grant, now being funded at about $2.4 billion by Congress, and the Community Services Block Grant, the remnants of the antipoverty program now being funded at $360 million.

The two programs combined were running at about $3.5 billion a year before Reagan came into office, but Congress cut them to a combined level of about $2.8 billion at his request.

In a related development, the Children's Defense Fund, in a new report called "Unclaimed Children," said two-thirds of the nation's estimated 3 million seriously disturbed children under 18 are not receiving services.

The report, which said some of the services are funded by the Social Services Block Grant, said only seven states are beginning to develop the comprehensive service programs needed for these children.

Such programs, it said, provide a wide range of treatments such as special classes, tutoring, counseling and psychotherapy, residential placements and institutionalization. It said the states are Florida, Georgia, Maine, Massachusetts, New Jersey, New York and North Carolina.

The report said that only 21 states have a special governmental unit to administer programs for disturbed children and that only about one-third of states even know how much they are are spending on disturbed children.

CDF said that about two-fifths of disturbed children hospitalized for emotional and behavioral problems would fare better being treated outside the hospital in other ways but that they could not because outside treatment facilities often do not exist. The number hospitalized was estimated at 83,000 several years ago.

The Social Services and Community Development programs have been popular with social welfare groups and state officials who must provide services, and the proposal to merge them with less money will probably encounter considerable opposition.

The National Association of Social Workers, asked Friday for comment on the proposal, said, "This is outrageous. It will mean the official end of the war on poverty and the loss of jobs and opportunity for poor people."

Under the Social Services program, the government makes grants to the states based on population. The money is used to fund day care centers and family day care homes, to provide homemakers for elderly and handicapped persons, and to fund referral services and protective services for abused, handicapped and emotionally disturbed children, transportation services for the elderly and handicapped and similar activities.

Under the Community Services Block Grant, funds are provided projects for the poor, such as local job creation efforts, services for the rural poor and poor persons to help staff local community action agencies.