WhatsApp users no longer have an excuse to ignore friends' messages after it introduced a new blue tick that tells senders if their messages have been read.
The messaging service introduced the new check to show when recipients had opened and read a message and makes clear if someone has read a message or their device has simply received it.
Until now users had assumed that the double tick meant recipients had read the message.
The change means that users will now find it a lot harder to convince someone they are chatting to that they had not read their message.
People chatting in a group chat will only receive the blue tick when everyone in the group has read the message.
Facebook, which bought WhatsApp earlier this year, already had a similar function and the move could be linked to the firm's acquisition.
The popularity of instant messaging services like WhatsApp and Apple's iMessenger have soared in popularity in recent years and overtaken SMS as the world's favourite way to send messages.
Research company Informa found almost 19 billion messages were sent on chat apps in 2012, compared to 17.6 million texts.