Google's Chrome browser will start blocking some internet adverts that use Adobe's Flash technology, from Tuesday.
Google said it had taken the step to help web pages load faster and preserve battery life on mobile devices.
It has introduced tools that help people convert Flash adverts to ones that use an alternative technology.
Many technology companies have turned against Flash in recent months, saying it slowed down web-browsing and was a security risk.
The option to block or pause Flash-based adverts was added to a test version of Chrome earlier this year.
The change was aimed at all add-ons for the Chrome browser that were hogging resources.
Many of these handle content, such as adverts or browsing aids, not directly connected to the main page being viewed.
This option has now been switched on by default and could mean that many adverts built with Flash do not run. Clicking on the advert will make it run.
Anyone who wants Flash adverts to run by default will be able to turn off the option.
The change could mean many organisations have to rework their advertising content if it is not served up to web pages using Google's AdWords system that automatically converts Flash adverts to HTML5.
HTML is the basic language of the web and is used to describe how web browsers should display text, images and video.
In early July, security problems with Flash led to it being blocked by default by Mozilla - the organisation that develops the Firefox browser.
Flash has been used to make many online banner adverts, pop-ups and video ads since the early days of the web.
However, the technology industry has steadily been turning against Flash, especially as many criminal hackers target it or create malicious ads to hijack victims' computers.
Apple was one of the first to block the software on portable devices in 2010.