Not content with a strongly-worded rebuttal and strategically-placed digital ads, the NFL is taking its fight against the New York Times a step further, demanding retraction of a story the newspaper published last week about the league’s allegedly incomplete concussion research and alleged ties to Big Tobacco.

Brad S. Karp, the NFL’s legal counsel, wrote that the story, which was headlined “NFL’s Flawed Concussion Research and Ties to the Tobacco Industry” is “false and defamatory” in a letter to the Times that was obtained by Politico. Karp, of Paul, Weiss, Rifkind, Wharton & Garrison, writes: “Like the ‘Big Tobacco’ charge, the thesis that the NFL manipulated concussion data to skew research on which the [l]eague continues to rely is false and defamatory and not supported by the facts set forth in the story.”

Karp hints that legal action may be in the offing, too, by requesting that “the Times reporters and editors who worked on this story preserve their notes, correspondence, emails, recordings and work papers and all other electronic and hard copy documents generated or received in connection with this work.”

In the letter, Karp goes on to note that the Times was provided with “extensive evidence” before the story was published and accuses the newspaper of using a “sensational headline” to tie the league to the tobacco industry in the absence of sufficient evidence.

“The Big Tobacco smear is especially pernicious and unfair because the truth is that there are few institutions in American life that do not have some intersection with the tobacco industry at some point, however devoid of meaning,” Karp writes. He goes on to add that a search of the 14 million documents from tobacco litigation archives revealed “significant ‘connections’ between the Times and the tobacco industry.”

The Times story reported that the league’s concussion research from 1996 to 2001 was flawed and that as many as 100 concussions, including those to players as well known as Troy Aikman and Steve Young, went unreported. Karp notes that the headline was later revised, but Times Sports Editor Jason Stallman points out that the headline in the print edition was not the same because headline specifications are not the same for print and online products.

“There is not a single fact in the Times article that supports the allegation that the NFL intentionally concealed concussion data in a manner ‘parallel to tobacco research,’ as the revised Times print headline asserts,” Karp writes. “To the contrary, the fact that the studies relied on a data set that was not a complete count of all concussions in the NFL — the central factual assertion in the story — was repeatedly and expressly disclosed in the studies themselves and counting all concussions was not the purpose of the studies in any event. Nor does the story explain why the allegedly ‘omitted’ data were necessarily significant to the purpose of the studies or their results.”

The letter appears to be part of a more aggressive strategy by the NFL. The league responded immediately when the Times story was published March 25, issuing a 2,500-word rebuttal of a number of facts. And the Times fired back, taking the unusual step of tweeting a defense of its story. The NFL even purchased ads on the Times’ website that appeared alongside the story it was protesting. The Wall Street Journal reported that by mid-afternoon Friday, its “reporters could no longer find the ads on the site. A Times spokeswoman said the ads were still in circulation, however.”

The Times responded that it has no plans to retract the story. “To date, we note, the NFL has not found a single factual error in our reporting,” Stallman told The Post in an email, “and now appears to be trying to divert attention away from the article itself.

“Any comparison between the Times and the NFL is absurd.

“We see no reason to retract anything.

“The NFL apparently objects to our reporting that the league had ties to the tobacco industry. But, as the article noted, a co-owner of the Giants, Preston R. Tisch, also partly owned a leading cigarette company, Lorillard, and was a board member of both the Tobacco Institute and the Council for Tobacco Research, two entities that played a central role in misusing science to hide the risks of cigarettes. Also, the NFL and the tobacco industry shared lobbyists, lawyers and consultants.

“The NFL also apparently objects to our reporting that the studies produced by the league’s concussion committee were more deeply flawed than previously understood. The league has always maintained that the studies were based on a data set that included every concussion that was diagnosed by a team doctor. In fact, our reporting showed that more than 100 such concussions — including some sustained by star players — were not included in the data set, resulting in inaccurate findings.”

This post has been updated to clarify why the Times’ headline differed in print editions.