Sensitive personal data including names, addresses and postcodes has been made publicly available on Parcelforce's website.
According to the BBC, the data could seen by anyone using Parcelforce's mail-tracking service. Entering a delivery number into the 'Track and Trace' function on the website displayed information including addresses and signatures of unconnected deliveries.
Parcelforce said the error occurred after work on its computer system but has no been rectified. However, the company will still face an investigation by the Information Commissioner's Office (ICO).
"We will be contacting Parcelforce to establish how this security breach occurred and to find out what steps it will be taking to ensure that such a breach cannot happen again," a spokeswoman said.
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