The Oakland City Council has authorized a multimillion-dollar antitrust lawsuit against the NFL and the Raiders over the team’s impending move to Las Vegas — legal action that Coliseum officials said could result in the team leaving Oakland at the end of the upcoming season.

Oakland Councilman Noel Gallo said Tuesday that the lawsuit is expected to be filed and announced within days, probably before the Raiders’ home opener at the Coliseum on Monday night against the Los Angeles Rams.

A second City Hall source told us the city attorney’s office was preparing to file the antitrust suit, and is awaiting contract language sign-offs from three outside law firms that will handle the litigation.

The outside attorneys have agreed to cover all the up-front costs of the suit, which will likely seek millions of dollars in damages for the team’s exit. In exchange, the outside firms would collect a portion of whatever dollar damages they are able to extract from the Raiders and the National Football League.

Gallo predicted that a lawsuit against the league and Raiders could result in winning damages of up to $500 million.

“It’s going to happen,” Gallo said of the suit, calling it “absolutely a good thing.”

The Raiders, however, do not share Gallo’s opinion.

According to Oakland-Alameda County Coliseum Authority Executive Director Scott McKibben, who has been working to finalize the Raiders’ latest lease extension, team executives warned him that they won’t sign any agreement if the lawsuit is filed.

“They told me we might as well take the contract and rip it up,” McKibben said Tuesday.

The Raiders, who did not return our call seeking comment, are under a year-to-year lease with the Coliseum Authority.

The team has been working on a new deal to play at the Coliseum next season and possibly the one after that if their Las Vegas stadium isn’t ready. According to sources, that deal — expected to be voted on by the authority’s board within a matter of weeks — would net the city and county $3.7 million next year and $5 million the following year, if the team extended its stay.

Oakland’s decision to sue was made within the past week without the approval of either the Coliseum directors or the Alameda County Board of Supervisors.

And not everyone is on board.

“It makes absolutely no sense,” said Oakland-Alameda County Coliseum board member Ignacio De La Fuente, who was an Oakland City Council member in the 1990s when the city voted for a deal to bring the Raiders back to the East Bay from Los Angeles.

De La Fuente said the city may be opening itself up to a countersuit by the Raiders, one that could wind up costing the city big bucks in the end.

“I think the mayor (Libby Schaaf) and the City Council really are just looking to justify themselves for not having done anything to retain the Warriors — and the A’s, for that matter,” he said.

But Gallo doesn’t see the risk.

“The attorneys needed a client, and Oakland is the client that has been signed up,” he said. “And hopefully we can get the county to join in.”

However, Alameda County Supervisor Nate Miley, who also chairs the authority, said Tuesday the county likely doesn’t have any interest in signing on to the suit — or, for that matter, staying in the sports business.

He said the county supervisors, who are on recess until Sept. 17, still hope to sell the county’s half-interest in the Coliseum site to the city, and any “lawsuit might prolong our entanglement with the sport team.”

Still, Miley said he personally supports the city’s decision to file a lawsuit, even if it does prompt the Raiders to exit the Coliseum early.

“I think there’s a greater upside to the city pursuing this ... and they are in a good position to force the NFL to come back with damages as a result of the decision to move the Raiders to Las Vegas,” he said.

Schaaf was on a plane to the East Coast on Tuesday and was unavailable for comment. However, her spokesman, Justin Berton, said, “The mayor’s office cannot comment on anything that was discussed or may have been discussed in a closed session of City Council.”

However, Schaaf has privately been known to be on the fence, expressing concern that she would support the suit only if it wouldn’t cost the city or result in tying up any future development plans for Coliseum property.

Oakland City Attorney Barbara Parker did not return calls seeking comment either.

Oakland and Alameda County still have a roughly $75 million bond debt for renovating the stadium to bring the Raiders back from Los Angeles.

Oakland Council President Larry Reid didn’t return repeated calls seeking comment. In March, when the outside law firms pitched the idea of a no-cost lawsuit, Reid questioned participating, saying: “I don’t see how you get a judge to force the NFL or the Raiders to pay off that bond indebtedness when it’s our debt.

“And if those who want us to join this suit think the NFL is going to commit to awarding another football franchise to Oakland, I don’t think they live in the real world,” Reid said.

Back in March, representatives from a pair of law firms — Berg & Androphy in Houston and Weil, Gotshal & Manges of New York — met with city officials to discuss filing the suit.

Jim Quinn, an attorney at Berg & Androphy who has successfully sued the league over both player free agency and the 2011 players’ lockout, told us at the time, “I think they have a valid lawsuit, and they ought to pursue it, because it’s the only way to get the attention of the NFL going forward.”

San Francisco Chronicle columnists Phillip Matier and Andrew Ross appear Sundays, Mondays and Wednesdays. Matier can be seen on the KPIX TV morning and evening news. He can also be heard on KCBS radio Monday through Friday at 7:50 a.m. and 5:50 p.m. Got a tip? Call (415) 777-8815, or email matierandross@sfchronicle.com. Twitter: @matierandross