A Senate amendment to block President Reagan's tough new antiabortion regulations for the federal Family Planning Program has been killed by House-Senate conferees handling the Department of Health and Human Services appropriations bill.

Instead of a specific provision sponsored by Sen. Lowell P. Weicker Jr. (R-Conn.) barring HHS from publishing the new rules, the legislators substituted conference-report language declaring that "the Senate conferees and some House conferees believe that changes to existing law must be achieved through adherence to the constitutional process, and express their disagreement with the executive bypassing that process through a regulatory device."

Observers expressed doubt that this language has the legal force to bar final publication of the regulations, which is expected shortly.

Under the $146 million-a-year Family Planning Program, the federal government makes grants to states, local government units and private nonprofit groups to operate family-planning clinics, which provide contraceptive, health and fertility services.

Such clinics are barred from using any of the federal funds for performing abortions. But they are required under longstanding rules to inform a woman facing an unintended pregnancy that her options include abortion, keeping the baby or giving the infant up for adoption. If she requests, counselors must refer her to an outside clinic, not funded by the program, where she can obtain an abortion.

The National Right to Life Committee and other antiabortion groups have argued that the existing rules allow federally funded planning clinics to "funnel" women to abortion clinics.

The White House earlier this year ordered HHS to change the referral rules.

Under the president's proposed regulations, family planning clinics receiving federal support would be forbidden to inform a woman that abortion, keeping the baby or adoption are all options for handling an unintended pregnancy. Providing a specific list of outside abortion clinics also would be forbidden.

If an organization runs a federally funded clinic but also operates a separate clinic where it performs abortion or abortion-related activities with separate funds, it must keep the two physically separated under the president's proposed regulations.

Rep. William H. Natcher (D-Ky.), chairman of the House Approproriations subcommittee that handles the bill, refused in a session Wednesday to accept the amendment blocking final publication of the president's new rules. With Rep. Henry J. Hyde (R-Ill.), an abortion foe, sitting in as a consultant, Natcher said he would have to put the matter up to a House vote, which several observers said probably would have resulted in House rejection of the Weicker amendment.

Douglas Johnson, legislative director of the National Right to Life Committee, which favors Reagan's new regulations, said the conference language "has no legal weight" and would not bar the administration from publishing the rules shortly in final form.

Scott Swirling, executive director of the National Family Planning and Reproductive Health Association, which backed the Weicker amendment, said his organization plans to seek court action to block the regulations when they are published on grounds that they will deprive women of information from a federally funded clinic on the full range of medical options relating to an unintended pregnancy.