It is not known how long the outage will last. Website DownDetector showed a massive spike in people reporting the app, owned by Meta, not working for them on Tuesday morning.
WhatsApp is one of the most popular messaging platforms in the UK and around the world, and is estimated to have more than two billion active users globally.
The app would allow people to type messages to friends and then press send. But they then became stuck, never actually delivering.
It means that users are likely sending and being sent messages without them actually being delivered – potentially leading to problems.
The issues led to a huge spike of reports on outage tracking website DownDetector, with users around the world reporting problems.
The issues appeared to begin around 8am UK time.
WhatsApp does not maintain any official outlets to update users on outages. Unlike the rest of Meta, it does not run an official status page, and it has a Twitter account intended to document problems but has not used it since 2014.
It also runs official Facebook and Twitter accounts, but neither had been updated in response to the outage.
WhatsApp has more than two billion monthly active users, and is the third biggest social platform after Facebook and YouTube.