San Francisco — Alphabet's Google will begin featuring ads on the homepage of its mobile website and smartphone app later in 2019, it said on Tuesday, giving the search engine a huge new supply of ad slots to boost revenue.

Google will also start placing ads with a gallery of up to eight images in search results, potentially increasing ad supply further. The ads will appear on Google pages and apps globally.

The changes come as choppy revenue growth prompt questions from some Alphabet investors about whether services such as Amazon.com  and Facebook's Instagram are drawing online shoppers and, in turn, advertisers away from Google.

Google executives said on Monday the latest features were a response to how users behave, not competition.

The company wants to make it easier for users to discover and buy new products because they shop in brief spurts while watching TV or sitting in the bathroom, said Oliver Heckmann, vice-president of engineering for Travel and Shopping.

"It actually changed with mobiles and what users expect from an online service like Google," he said.

The ads on the Google home page appear on what the company calls its Discover feature, a Facebook-style news feed that users swipe through to view an algorithmically personalised set of links to articles, videos and other online content.

Google has been testing ads on Discover since 2018, when it said more than 800-million people were using the feature monthly. On the smartphone app, the ads will appear on the main screen.

The gallery ads are part of a Google effort to make search results more visual, the company said. Ads with images are expected to garner more clicks, which could lead them being shown in more results, executives said.

Google — which announced the changes at its Marketing Live conference  for advertisers — also said it would begin showing personalised content on its Google Shopping home page. That offers only a search box in most countries at present.

The company will continue to use the Shopping service to take orders for in-store collection or delivery from a variety of shops. But it will phase out its Google Express delivery brand in favour of Google Shopping.

Google first launched a more robust shopping page in India in 2018.