The Carter administration's hospital cost control bill will be met with the "united opposition" of both hospitals and physicians, the president of the 6,000 - member American Hospital Association said yesterday.

The hospital association's opposition to the bill, announced at a press conference yesterday by president J. Alexander McMahon, although not unexpected, underscored the difficulties likely to face the legislation as it moves through Congress.

The bill was introduced in the Senate yesterday by Sen. Edward M. Kennedy (D. Mass.), chairman of the Health Subcommittee, with Sens. Wendell R. Anderson (D-Minn.) and William D. Hathaway (D-Maine) as last minute co-sponsors. The administration was apparently unable to find a Democratic co-sponsor on the Senate Finace Committee, which will have primary responsbility for action on the bill.

In the House, both sponsors of the bill - Ways and Means Health Sub-committee Chairman Dan Rosten-kowski (D-Ill.) and Commerce Health and Environment Subcommittee Chairman Paul G. Rogers (D-Fla.) - expressed reservations about the bill a they introduced it on Monday.

In other reaction; AFL-CIO President George Meany contrasted the administration proposal, which he said will on proposal, which he said will "only restrain and not stop the excalation in health care costs." With a bill sponsored by Kennedy, supported by the AFL-CIO, which "contains the most stringent and effective cost controls of any national health insurance proposal."

The only part of the administration proposal Meany endorsed without qualification was the provision to set a $2.5 billion ceiling on capital expenditures for construction and new facilities and the bill's encouragement of pre-paid health maintenace organizations.

The administration's two-part proposal includes a limit on the amount that most hospitals can increase their revenues each year. Although some exceptions would be allowed, most hospitals would be limited next year to no more than a 9 per cent increase from Medicare, Medicaid, Blue Cross, other insurance plans and from direct patient payments.

The underlying assumption of the administration plan is that hospital costs, about $55.7 billion in 1976 and increasing at a rate of about 15 per cent annually, be checked quickly only by imposing an outside limit. With government and private insurance plans paying 90 per cent of hospital bills, neither hospitals, doctors nor patients have any direct incentive to restrain expenditures, according to administration officials.

A spokesman for the Blue Cross Association, whose individual local members would bear a heavy responsiblility for enforcing the administration's program, said the association supports the $2.5 billion limit on capital expenditures - roughly half what was spent in 1976.

But the spokesmon said that Blue Cross would prefer a "more flexible" approach toward improving hospital efficiency than an across-the-board limit on revenues.