Google has told publishers it will give them at least six months to prepare for a new ad-blocking tool the company is planning to introduce in its Chrome web browser next year, according to people familiar with the company’s plans.

The new setting, which is expected to be switched on by default within the desktop and mobile versions of Chrome, will prevent all ads from appearing on websites that are deemed to provide a bad advertising experience for users.

To help publishers prepare, Google will provide a self-service tool called “Ad Experience Reports,” which will alert them to offending ads on their sites and explain how to fix the issues. The tool will be provided before the Chrome ad blocker goes live, the people familiar with the plans say.

The company hasn’t yet announced or acknowledged its plans, but has briefed publishers, agencies and advertisers about its intentions in recent weeks. It is possible Google’s plans or release timing could change.

Unacceptable ad types include those identified by the Coalition for Better Ads, an industry group made up of various trade bodies and online advertising-related companies that says it aims to improve consumers’ experience with online advertising.

The group’s initial list of unacceptable ad types, released in March, included pop-ups, auto-playing video ads with sound and “prestitial” ads that count down before displaying content. Google is a member of the group, alongside fellow ad giant Facebook, and Wall Street Journal parent News Corp.

As described to publishers, Google’s feature will block all ads on sites that have a certain level of unacceptable ads. Publishers have been advised to ensure their sites are compliant if they want their ads to be displayed.

While blocking pesky ads is appealing to consumers, it also threatens a vital source of revenue for publishers. Some media companies are wary of Google’s tool impacting which of their ads make it through to users. Still, other publishers would actually welcome Google’s entry into ad blocking if it can help filter out the spammy, irritating ads they believe have driven users to attempt to avoid or block all online ads.

Google is describing the new feature as an advertising “filter” as opposed to a “blocker,” and compares it to existing features in Chrome designed to block pop-up windows and to offer users warnings before they visit pages with harmful software known as malware.

The ad-blocking step might seem counterintuitive given Google’s reliance on online advertising revenue, which accounted for 88% of Google-parent Alphabet Inc.’s more than $90 billion in revenue last year.

The move could be a defensive one. Consumers who are frustrated with internet ads that clog up pages and slow down browsing have been turning to ad blockers in recent years. Some 26% of U.S. web users now employ the software on their desktop devices, according to the Interactive Advertising Bureau.

By making its own tool available by default in Chrome, Google may be hoping to quell further growth of blocking tools offered by other companies, and therefore have more control over which ads get blocked online, people familiar with the company’s plans said.

Google’s Chrome dominates the internet browser market with nearly 60% share across mobile and desktop devices, according to internet statistics provider NetMarketShare.

In addition to the ad blocking tool, Google is also pitching publishers on a new tool it is calling “Funding Choices,” which is designed to combat the effects of ad blockers offered by other companies. If publishers enabled that feature on their sites, users with ad blockers turned on would be presented with messaging prompting them to disable their blockers to access the content, or to pay for a “pass” to remove advertising entirely.

It is unclear if passes will be offered for individual sites, or for a bundle of sites that opt-in to the program. People familiar with the program described it as an updated version of an earlier product called Google “Contributor.”

Write to Jack Marshall at Jack.Marshall@wsj.com