The number of current coronavirus cases in Wales is almost twice as high as previously thought after it was revealed 11,000 positive tests were missing from official figures.
Public Health Wales (PHW) said a maintenance on an IT system led to “significant under-reporting” of cases between the 9 and 15 December.
The number of new cases previously reported for those dates stood at 11,911 but the real figure is almost double at 22,911.
PHW said all those who had tested positive were informed correctly despite the tests being missing from official figures.
Welsh first minister Mark Drakeford told BBC Breakfast: “This was planned upgrading of the computer system, none of the data is missing, everybody who had a positive test in Wales was told that last week, everything was uploaded.
“But the figures do demonstrate just how serious the position here in Wales has become and underlines why we made the decisions yesterday, both in the lead-up to Christmas, during Christmas, and once Christmas is over.”
Plaid Cymru health spokesman Rhun ap Iorwerth said the news “raises questions” and “rocks trust” in the government.
He told BBC Radio Four’s Today programme: “I think we’re heading into a period that is about as tight as it can get starting from Christmas Eve.
“I must say when the first minister announced yesterday that from the 28th there would be a tightening of restrictions, and I could see the figures as they were, one wondered why not do things now in terms of stepping up support for areas most heavily affected, but this does put a new slant on it again.”
He added: “This is a serious bulk of figures being added to the total today and it raises questions about the way data is analysed.
“It raises questions again about the interaction between what is controlled in Wales and the Lighthouse laboratories, and it’s one of those things that again sort of rocks trust in what’s happening and the data that’s presented to us.”
The development comes ahead of a review of the tier system on Thursday, with reports suggesting the number of people living under the toughest restrictions could increase.
Boris Johnson insisted he did not want to “cancel” Christmas, but said people should prepare for a “smaller, safer” festive period amid fears about the spread of coronavirus.
Confirming the so-called “Christmas window” allowing three households to join together between December 23 and 27 would go ahead, Johnson said people would be left to make individual judgments on whether Christmas celebrations were worth the risk, with warnings to avoid elderly relatives and a recommendation to isolate beforehand.
The government had wanted a UK-wide approach to Christmas, but Wales will now legislate to restrict mixing to two households – and will go into a tougher lockdown on December 28.
All parts of the UK are issuing tougher guidance, with a joint statement by the governments of the UK, Scotland and Wales stating: “The safest approach may be not to form a Christmas bubble.”