Legislation to require strong health warnings on smokeless-tobacco packages and print advertising seemed to have a clear field to Senate passage this week when it was blind-sided by a bill backed by the National Football League.

In a surprise development, Sen. John C. Danforth (R-Mo.) announced his intention to attach the sports bill as an amendment to the tobacco bill, which raised the likelihood of a filibuster against the combined measure. His plan sidelined the tobacco bill, at least temporarily.

The House passed the smokeless-tobacco bill by voice vote Monday, implementing a fine-tuned compromise worked out over a period of months by an unusual alliance of the Smokeless Tobacco Council, a trade association, the Coalition on Smoking or Health and key legislators. In addition to requiring three separate warnings, the bill would ban television and radio ads for smokeless-tobacco products, which are largely moist snuff and chewing tobacco. An estimated 22 million Americans use the products.

Council President Michael J. Kerrigan and coalition Director Matthew L. Myers said yesterday they had expected the Senate to adopt the bill and send it to President Reagan Monday afternoon or Tuesday.

The council denies charges that the products cause oral cancer, gum disease and loss of teeth in some users, but supports the compromise because numerous state legislatures are poised to require conflicting warnings. Most ominous for smokeless-tobacco manufacturers, the Massachusetts Department of Health will require an addiction warning if a federal law is not in effect by April 1.

Hitches began to develop shortly after the House passed the bill, according to the separate accounts of Kerrigan, Myers and Senate staff members. All said in interviews that they are concerned opponents will try to build momentum against the bill during the Senate recess, which begins Friday and runs through Feb. 17.

The initial difficulties involved three Republican senators: Jesse Helms (N.C.), whose tobacco state grows no smokeless-tobacco plants, Orrin G. Hatch (Utah), a sponsor of the bill and chairman of the Senate Labor and Human Resources Committee, and Danforth, who has been angered by earlier threats of a filibuster if his sports bill were brought up on the Senate floor.

The bill would grant the NFL and basketball, hockey and soccer leagues antitrust exemptions in franchise relocation and revenue sharing.

The bill, approved by Danforth's Senate Commerce Committee April 2, has languished in the face of a threatened filibuster by Sen. Albert Gore Jr. (D-Tenn.), who would be supported by at least two senators.

The tobacco bill's advocates had seen Helms as a possible adversary, mainly because the required warnings would be more dramatic than those on cigarettes.

The word "warning" would be enclosed in a circle that points to the text inside the circle.

On Monday, Helms said, he made a routine request to Hatch to delay a Senate vote overnight so he could become familiar with the bill and its possible "applicability down the road in terms of cigarettes" and possible impacts "on the livelihoods of a couple of hundred thousand people in my state." He did have "some heartburn about the circles and arrows," he said. But on Tuesday he told Hatch he would not stand in the bill's path.

Meanwhile, Danforth and Commerce Committee aides had a pre-arranged meeting with representatives of the NFL.

Jay E. Moyer, the NFL's executive vice president and counsel, said the strategy of making the sports bill an amendment to the tobacco bill had been "formulated by the Commerce Committee."

He denied suggestions by tobacco-bill proponents that the idea originated with either the NFL or Martin B. Gold, a partner in the Washington consulting firm of Gold and Liebengood. The firm represents the NFL, the Tobacco Institute -- the cigarette industry's trade association -- and the Outdoor Advertising Association.

Neither Danforth's press secretary nor the Commerce Committee's general counsel, Walter B. McCormick Jr., returned a reporter's phone calls yesterday.

Gold, who attended the meeting with Danforth, said he had suggested the sports bill might fail unless it were attached to some other piece of legislation within a few months.

But the idea of attaching it specifically to the tobacco measure came from the committee staff, he said.

Howard Liebengood, Gold's partner, said: "There is no connection between the (firm's) representation of the cigarette industry and the representation of the NFL. In other words, Gold handles the NFL, I handle the tobacco account. I did not know about the Danforth proposal until it was proposed."

Last night, supporters and opponents of the tobacco bill were trying to get Danforth either to abandon or stay with the proposed linkage to the sports bill.

Irene Howard, an aide to Hatch, said the senator had told Danforth that the tobacco bill was important and that he was going to try to have it passed before the recess starts Friday. As of last night, it wasn't known whether Danforth would try for the linkage.