Google is warning users to check for suspicious activity on their accounts (Photo illustration by Chesnot/Getty)

As Google prepares to delete millions of Gmail accounts for security reasons, the company also warns users to look out for signs they have been hacked.

From December 1, Google will be deleting accounts that have not been used for at least two years, warning they are more vulnerable to attack. This is because they are less likely to have two-step verification set up, and they have older passwords which may have been leaked onto the dark web in a previous hack.

However, accounts in regular use are still at risk from hackers, and Google has raised three ‘red flags’ to look out for.

The first, it says, is someone other than the account user changing account information, including the password or recovery phone number. While the first is easy to spot, many may not regularly check other account details – and by changing the recovery phone number, a hacker can take charge of your email.

In other cases, a hacker may delete your account entirely after use, or find another way to stop you from signing in.

If you notice any of the above and think your account has been signed in to by anyone else, the first thing to do is change your password immediately – not just for Gmail, but any apps and sites that you have used the same password for or log in to using your Gmail account.

Use two-step verification to keep your account safe (Picture: Getty/iStockphoto)

You must also change the password on your Google password manager.

Next, check for suspicious activity.

How to review your Gmail account activity

Go to your Google account

On the left navigation panel, select Security

On the “Recent security events” panel, select Review security events

Check for any suspicious activity:

If you find activity that didn’t come from you: Select No, it wasn’t me. Then, follow the steps on the screen to help secure your account

If you did the activity: Select Yes. If you still believe someone else is using your account, find out if your account has been hacked

Next, check which devices are using your account

In the security panel, click on ‘Your devices’ and select ‘Manage devices’

Check for any devices you don’t recognize.

If you find a device that you don’t recognize: Select Don’t recognize a device? Then, follow the steps on the screen to help secure your account

If you recognise all the devices, but still believe someone else is using your account: Find out if your account has been hacked

Once you have checked both your account and devices and are happy it is secure, be sure to turn on two-step verification if you haven’t already – this means that even if your password is stolen, your account should still be secure.

MORE : Three-week warning for anyone with a Gmail account

MORE : Now anything you post publicly online could be used to train Google’s AI

MORE : In praise of the password – the key to your digital kingdom

Is your account safe?