WhatsApp is set to add voice calls to its app for the first time in a bid to take on Skype and other messaging firms.

The move is expected to open up major new markets for the firm Facebook bought for $19bn.

It will allow users of the app to make voice calls to contacts in the same way they would send a message.

The screens of the feature were recreated by a Dutch web site from code in the app

Dutch site AndroidWorld uncovered the screenshots in test code.

The 'screenshots' above have been created by AndroidWorld.nl's Sander Tuit, who extracted layout files, images, and text from the raw code of the app hosted on WhatsApp's website.

It is expected the feature will work exactly like text or picture messages.

Users will be able to use either their handset or a bluetooth headset to make calls.

A phone-like page will show calls made an received.

Earlier this month the firm, the firm updated its app to make it harder to avoid WhatsApp messages, or at least claim that you haven't seen them.

The Facebook-owned app will display two blue ticks in the bottom right-hand corner of an individual message to show that it has been read.

And clicking on the message will show you the precise time the recipient opened and read it.

The app update is available across all mobile platforms. From now, a single grey tick means the message has been sent, a double grey tick means it has been delivered, and two blue ticks mean it has been read (pictured)

More information about the features can be found on WhatsApp's FAQ page.

Before the update, there were only two types of tick - a single grey tick meant the message had been successfully sent, and a double grey tick meant it had been delivered.

Now, when a message has been sent, delivered and read, the double grey tick turns blue.

In a group chat, the second ticks appear when all participants in the group have received the message.

WHATSAPP'S ENCYPTED MESSAGES WhatsApp, the global messaging system owned by Facebook, has begun using a new encryption program aimed at protecting users' conversations from unwanted surveillance and snooping. The messaging service, which has 600 million users, added the new encryption to its last update for Android smartphones, according to Open Whisper Systems, the San Francisco-based software group. Privacy advocates say WhatsApp's decision to incorporate so-called 'end-to-end' encryption will help users communicate without the fear of someone reading their messages. WhatsApp cofounder Jan Koum has said he's deeply committed to user privacy because he grew up in the Soviet Union during the 1980s, when his mother and other adults routinely assumed that authorities eavesdropped on their phone calls. Advertisement

The two blue ticks only appear when all participants in the group have read the post.

To see what time the messages were read users can now press and hold down on a particular post.

Selecting the Message Info option reveals the time the message was read, and in the case of group messages, what time each member read it.

WhatsApp has rolled out the changes to all variations of its app, across iOS, Android, Windows Phone, Nokia S40, Nokia S60 and BlackBerry 10.

Users need to update the app on their devices to see the changes.

However, these changes have divided opinion, especially on Twitter.

Some users are concerned the update makes it harder for them to ignore others, and makes it easier for people to keep tabs on them.

As @the_baba1 wrote: 'So WhatsApp now shows blue ticks to indicate a message has been read. Now stalk with more vengeance.'

Twitter user @carpocarp said: 'The WhatsApp blue tick is going to ruin friendships,' while @archielbar added: 'Those blue ticks will be the death of some of us!'

But not all Twitter users were concerned about the update - @tnicole91 explained: 'I don't get why some people are stressed about this new WhatsApp blue tick.

'For me, if I don't feel like replying, I just don't.'

In a group chat, the blue ticks only appear when all participants have read the message.To see what time the messages were read, users can now press and hold down on a particular post and select the Message Info option. In the case of group messages, this reveals what time each individual member read it (pictured)

WhatsApp explained the changes and revealed what the new check marks look like on an official blog post

In an official blog post, WhatsApp tried to reassure users who may be concerned about why a person hadn't received or read their messages.

'If you only see a single check mark next to your message, there is likely nothing wrong with WhatsApp or your phone,' explained the developers.

'There may be several reasons why your message has been sent, but not delivered to your chat partner such as their phone might be off, they could be sleeping, especially if they live in a different time zone, they might be experiencing network connection issues or they might have seen the notification on their screen, but did not launch the app - especially common if the recipient uses an iPhone.'

It is not currently possible to opt out of this feature.

WhatsApp is owned by Facebook, which has a similar feature on its Messenger app. When a message is opened and read, a tick appears in the message list and the time it was read appears in the bottom right-hand corner of the post.