The miltibillion-dollar emergency farm bill, already facing a house floor fight and threatened presidential veto, was confronted by a new obstacle yesterday in the Congressional Budget Act.

As the House-Senate conferees held a final meeting to sign their agreement on a bill to increase price and income supports for grain and cotton farmers, Rep. Thomas Foley (D-Wash.) House Agriculture Committee chairman, said the House parliamentarian has adviced that the bill violates the Budget Act.

The farm measure as written by the Senate and accepted by House conferees would authorize higher support levels during this next crop year in proportion to the amount of acreage the farmers take out of production.

But the Budget Act forbids Congress to vote before May 15 to authorize spending for the fiscal year beginning next Oct. 1. This is to give Congress time to pass the first budget resolution setting overall spending targets before it starts to hand out the money.

Foley said there are two ways to protect the measure from a point of order by any member which would kill it. One would be for the House to vote on the conference report under a procedure that suspends the rules but requires a two-thirds vote to pass. Since it is not at all certain the bill can master a majority vote in the House with its big-city delegations, this course is not likely to be followed.

The other way is to ask the House Rules Committee to report out a resolution waiving a point of order that the farm bill violates the Budget Act. Foley is expected to ask for such a rule, but the Rules Committee only once has voted such a rule over the opposition of the House Budget Committee.

Rep Robert Giaimo (D-Conn.), House Budget Committee chairman, said he is opposed to the bill and opposed to granting a waiver from the Budget Act. He said a sampling of opinion indicated a majority of his committee will support him.

House Speaker Thomas P. O'Neill (D-Mass.) hinted yesterday that farm bill supporters might have trouble getting the Rules Committee to vote a waiver.

If all else fails, the bill could be delayed until after May 15 and then brought up for a final vote without any problem. But by then, farmers across the country will have planted their crops without knowing whether they might be rewarded for diverting extra acres from wheat, corn and cotton, and President Carter is expected to veto it whenever it reaches him.

The administration opposes the bill as inflationary. The Agriculture Department still had no figures yesterday on its impact, but officials estimated the measure could raise retail food costs by 2 percent or more.

The Senate may take final vote on the conference report this week. The Budget Act issue could also be raised there but has not so far. The House will not be able to act until next week at the earliest.