The Welsh Government has "sincerely apologised" after 13,000 letters intended for people most vulnerable to coronavirus were sent to the wrong addresses.
More than 80,000 people with serious underlying health conditions should have received a "shielding letter" from the chief medical officer, advising them to stay at home for 12 weeks.
The letters were meant to be delivered by 3 April, and also contain advice and information regarding how those with no-one else to support them access medication and essential items such as food.
Plaid Cymru’s local government spokeswoman Delyth Jewell said: "This is a potentially disastrous mistake which could needlessly endanger lives."
Citizens eligible for the letter qualify for priority delivery slots from supermarkets, although there have been delays introducing the system.
The Welsh Government said local authorities and supermarkets had been provided with the correct addresses from the outset, with the latter using these for delivery slots.
Ms Jewell called for details of "what went wrong for this to happen in the first place".
A Welsh Government spokeswoman said: "Due to a processing error within the NHS Wales Informatics Service (NWIS), some shielding letters were posted to a previous address, where the intended recipient had recently moved.
"All letters have since been reissued to the correct addresses.
"We provided all local authorities with the correct details from the start of this process, and they have been directly contacting each person involved over the past two weeks."
She added: "We fully understand the concern this would have caused people and sincerely apologise for the mistake."
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