Facebook will no longer let users send and receive private messages within its main smartphone app, and will require that users install a separate messaging app, the company said.

Facebook began notifying users in France, England and several other European countries on Wednesday that beginning in a couple of weeks they will need to install the company's standalone Messenger app in order to send and receive messages.

In another development, Facebook said it is switching to fewer but bigger ads on the right-hand column of the desktop PC version of its website because users respond more frequently to them.

Spokesman Derick Mains said Facebook plans to eventually require that users in all countries install the Messenger app, though he could not provide a specific timeframe for the change.

The move is intended to ensure that users have a consistent and high-quality experience, a Facebook spokesman told Reuters. The free, standalone Messenger app is faster than the messaging service that's currently built into Facebook's primary mobile app. Facebook users can also access more features in the Messenger app, such as the ability to make voice phone calls.

"We have built a fast and reliable messaging experience through Messenger and now it makes sense for us to focus all our energy and resources on that experience," the company said in a statement.

Facebook faces increasing competition from a new crop of fast-growing mobile messaging apps, such as Snapchat and Line. It stunned observers by announcing plans in February to buy the popular WhatsApp messaging app for $19 billion.

Forcing users to install a separate messaging app on their phones could help boost the popularity of Facebook Messenger, but could also cause a backlash if consumers view Facebook's move as heavy-handed. A post on the technology blog Mashable on Wednesday was titled "11 Reasons why Pulling Messenger From Facebook Mobile is a Terrible Idea," and many people took to Twitter to post critical comments about the change.

Meanwhile, Facebook said the new right-hand display ads will be the same proportions as the so-called News Feed ads, which inject paid marketing messages straight into a user's stream of news and content. That will make advertising on Facebook simpler for marketers, since they will no longer need to use different images for different types of ads, Facebook said in a post on its Facebook for Business blog.

In tests of the new ad design, Facebook said it has seen up to three times more user "engagement" with the ads. The company did not provide specifics, but typically the term means some kind of user interaction such as clicking on the ad, posting a comment or resharing it.

Facebook did not specify how many ads will now appear in the right-hand column, though a sample image on the blog showed a single ad in the right-hand column. The company previously displayed as many as seven box-like display ads in the right hand column.

Facebook, which has more than 1.2 billion users, said some advertisers will begin seeing the new option later this month. The changes affect only the PC version of Facebook's website, which now accounts for less than half of its ad revenue as Facebook users and marketers increasingly favour the mobile version of its service. - Reuters