At least 11 companies have pulled ads in response to reports of harassment settlements.

An advertiser boycott against Fox News’ Bill O’Reilly picked up dramatic speed Tuesday in a rapid-fire reaction to the news that the outspoken host has been the subject of a succession of sexual-harassment allegations over the past 15 years.

At least 11 companies, including leading car, financial and pharmaceutical advertisers, confirmed that they had pulled ads or planned to shift upcoming ads away from “The O’Reilly Factor” in the wake of reports that O’Reilly and Fox had settled five complaints made by women who have worked with him at the news network since 2002.

Collectively the advertiser retreat, which seemed to grow in a chain reaction throughout the day, represented the most significant threat to O’Reilly seen during his long, successful and occasionally controversy-riven career at Fox. A similar but slower retreat by advertisers led to the demise of Glenn Beck’s highly rated Fox program amid boycott calls by African-Americans and Jewish groups in 2011.

Among the companies that confirmed they were suspending or removing ads from his program were the automakers Hyundai, BMW, Mercedes-Benz and Mitsubishi Motors; financial firms T. Rowe Price and Allstate Insurance; drugmakers Sanofi and GlaxoSmithKline; plus Ainsworth Pet Nutrition, the online marketing company Constant Contact and men’s apparel seller Untuckit.

A prolonged advertiser boycott of O’Reilly could prove financially painful to Fox. O’Reilly’s 8 p.m. news-discussion program is the highest-rated on cable, with an average 4 million viewers. It is also a tent-pole show upon which the rest of the conservative-leaning network depends. Fox counts on O’Reilly to generate an outsize share of its revenue and profit, which reached an estimated $1.7 billion last year, a record since the network’s founding in 1996.

On the other hand, it is not clear how long advertisers intend to stay away from O’Reilly, or what the short-term financial effect, if any, will be. Sponsors tend to buy cable ads in blocs, so removing an ad from one program usually means it is simply moved to another time slot, making little financial difference. What is more, cable-news channels derive the bulk of their revenue from licensing fees paid by cable and satellite operators, not advertising.

Despite knowledge of his multiple settlements, Fox and its parent company, 21st Century Fox, have stood by O’Reilly. The parent company last week re-signed the combative host to a new long-term contract that will pay him a reported $18 million per year and keep him on the air until at least 2020.

The advertisers who pulled out of O’Reilly’s program said they were responding to a New York Times story published Saturday that found the host and the network have separately or jointly paid at least $13 million to five female employees in exchange for their promise not to sue or publicly discuss their cases against O’Reilly. A sixth woman, former “O’Reilly Factor” contributor Wendy Walsh, said that O’Reilly had sexually harassed her, too, but that she did not intend to sue or seek compensation.

“While it’s hard to tell what the facts are, the allegations are disturbing,” said Donna Boland, a spokeswoman for Mercedes-Benz. “Given the importance of women in every aspect of our business, we don’t feel this is a good environment in which to advertise our products right now.”

Untuckit’s co-founder and chief executive, Aaron Sanandres, had a similar comment: “As a company in which more than two-thirds of our employees are women, we take sexual harassment claims very seriously. . . . In light of the disturbing allegations, we instructed our media buyer this morning to reallocate our ad dollars to other shows effective immediately.”

The harassment complaints against O’Reilly are on top of a separate wave of allegations against Roger Ailes, Fox News’s co-founder. The allegations surrounding Ailes came to light after Gretchen Carlson, a former Fox host, filed a lawsuit in July alleging that Ailes had pressured her for sex and demoted her when she refused.