The system sends messages from organisations including police forces and the Environment Agency during emergencies to those who have signed up to the service.
Users receive the alert as a text message and it also appears in their Twitter timeline with an orange bell.
The service was developed after the Japanese tsunami in 2011 when Twitter was widely used to find information.
“We know from our users how important it is to be able to receive reliable information during these times,” Twitter said when the service was launched in September in the US, Japan and South Korea.
The Environment Agency said in a statement: “We will only use Twitter Alerts to share urgent information about a significant risk to life or the environment as a result of flooding or an environmental incident.”
People who wish to sign up for Twitter Alerts will need to go to each organisation's Twitter feed and sign up for the messages. They will then receive text messages or push notifications when an alert is issued. More than 75% of Twitter users access the site from their mobile phone according to the company.
On a Twitter timeline the alert tweet will feature an orange bell at the end of the message designed to make it stand out from other tweets in a user's timeline.
The Foreign Office, the London Fire Brigade and the UK's 47 regional police forces have all signed up to use the system.
Twitter has 232 million monthly active users and the company says 500 million tweets are sent a day.