ChatGPT maker OpenAI has lobbed its boldest challenge yet at Google's dominance, unveiling Atlas - an artificial intelligence-powered web browser that launched globally on Wednesday.

The announcement immediately sent shares in Google's parent company Alphabet tumbling 2 per cent - wiping billions from its valuation - as investors digested the threat to Chrome's 3 billion-strong user base.

OpenAI chief executive Sam Altman declared Atlas a "once-in-a-decade opportunity" to reimagine how people interact with the web, arguing that tabs haven't sparked meaningful innovation since their introduction.

Altman said the browser's killer feature is an "Ask ChatGPT" sidebar that follows users across every webpage, eliminating the tedious copy-pasting that's become part of daily workflow. Users can open a movie review and instantly ask ChatGPT for a summary, or find a recipe and have the AI help order ingredients online.