The move follows tests of the feature in Australia and Thailand.

But Facebook is still describing the facility as being in "beta", indicating it may still tweak the service.

Last month, chief executive Mark Zuckerberg told shareholders he expected Messenger to become "a big business" within five years despite it not generating a "meaningful amount of revenue" yet.

He made a similar prediction for his other chat app, WhatsApp.

But despite changing WhatsApp's terms last year to permit businesses to send messages to users, Facebook has not yet revealed how it intends to deploy adverts within it.

Ads over bots

Messenger has more than 1.2 billion active users, according to Facebook.

For now, adverts will appear only in the app's inbox list of recent chats and not within the conversations themselves.

Tapping on an advert can bring up a relevant web page, allowing a retailer, for example, to sell their goods directly to the consumer.

But marketers can also opt to use the adverts to direct consumers to a conversation within Messenger instead.

Facebook's chief financial officer, David Wehner, has previously forecast that Facebook's advertising revenue growth rates would slow this year.

This was a result, he said, of a drop-off in the number of visits from desktop PCs to its products.

In addition, he said, the company wanted to avoid overloading its core Facebook and Instagram apps with paid-for content.

Introducing adverts to a product that had not featured them to date might therefore address investors' concerns.

But one company-watcher said the move represented a "plan B".

"When Facebook first spoke about trying to monetise Messenger, the whole idea was that it would be a big kind of customer service tool with people able to speak directly to brands to sort out problems," said Sarah Vizard, content editor of Marketing Week.

"The pitch was that companies could save money by setting up AI [artificial intelligence] chatbots to do this.

"There are some brands that are using chatbots on Messenger, but it's still seen as a bit of a gimmick.

"So, Facebook has been looking around for different ways to make money from Messenger and has obviously shifted its strategy a bit to think people will accept some ads within it."