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 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 O'Reilly's 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.

The list continued to grow late in the day; by early evening, CNN had pegged the number of companies pulling their ads at 18.

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. 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 blocks, so removing an ad from one program usually means it is simply moved to another time slot. 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 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 contract that will pay him a reported $18 million per year and keep him on the air until at least 2020.

The companies that pulled their ads said they were responding to a New York Times story on 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.

O'Reilly did not address the unfolding controversy on his program Tuesday.