A test email sent to more than 1.2 million employees has reportedly caused the NHS's email system to crash. Scott Barbour/Getty

A test email that was accidentally sent to more than 1.2 million NHS employees reportedly brought the organisation's entire email system to a standstill, staff members said. On Monday (14 November), frustrated NHS employees took to Twitter to complain about the test message that was believed to have been sent by an IT contractor in Croydon to every employee in the organisation.

The email triggered thousands of replies in response to the message that clogged up the email system and many users claimed on social media that the mail system had crashed. Peeved users urged fellow employees to stop responding to the email to avoid further flooding their inboxes with unnecessary mails.

One employee wrote that an estimated 186 million "needless" emails had been sent. According to NHS Digital, around 840,000 user accounts were affected as users reported issues with connecting and logging into the system as well as delays in sending and receiving messages from both internal and external sources.

"So essentially #NHSmail users have just carried out a DDoS [distributed denial of service] attack on themselves," one user wrote.

The Guardian reports that a message sent to NHS mail users called the email a "high severity service incident". The message said that an "issue with the distribution list" caused the test emails to be mistakenly sent to all users.

NHS Digital later said in a statement that a system "bug" was to blame for the chaos.

"Some users have experienced short delays in the NHS Mail system this morning," NHS Digital said. "A number of email accounts have been operating slowly. This was due to an NHS Mail user setting up an email distribution list, which inadvertently included everyone on the NHS Mail system. This was not the fault of the user and was due to a bug in the supplier's system."

"As soon as the issue was identified, our supplier disabled the distribution list so that no-one else could respond to it. We anticipate that emails sent before the distribution list was disabled will soon stop being received and that the issue will be resolved."