An extra 11,000 positive Covid tests in Wales have not yet been counted in the official figures because a computer system was down for maintenance, it has emerged.
The anomaly means the number of cases for the week up to 15 December is likely to be about double what had originally been thought.
Opposition politicians in Wales expressed deep concern, with one calling the news “staggering”.
The situation emerged after the Welsh government announced on Wednesday that the number of cases was so alarming that the country was to go into another lockdown.
It is not yet clear whether ministers were fully aware of the figures when it made the decision.
Currently, recorded figures for the the week of 9-15 December stand at 11,911. But Public Health Wales (PHW) said the 11,000 cases were missed from the figures because of “planned maintenance” of IT systems.
A total of 103,098 people have tested positive in Wales since the Covid crisis began, meaning the 11,000 extra positives – which will be added PHW figures from Thursday – will represent roughly one-tenth of the new total.
The additional 11,000 cases would mean recent cases of the virus are twice as high as previously thought.
PHW said the missing results were from tests carried out by the lighthouse laboratories, which were set up to help tackle the Covid crisis.
The Welsh first minister, Mark Drakeford, played down the issue. He said the computer upgrade had been announced in advance and it had been made clear that some cases would not be reported in the normal way.
Speaking on BBC Radio Wales, he said: “There is no computer problem. This is a planned and necessary upgrading. Nor have cases gone missing.
“Everybody who had a positive test was told about it; our TTP (test, trace, protect) knew about it. We will be adding those tests into the data. The data was never missing. The data was always waiting to be uploaded.”
Drakeford suggested the increase in cases was not a surprise to the Welsh government as it was hearing about them through the TTP system.
He pointed out a record number of tests were being carried out. Two weeks ago tests rose from 75,000 to over 100,000 and last week there were 120,000 tests. “If you are testing more people you will identify more positive cases.”
Butthe revelation increases pressure on the government, which has been criticised for how, on Wednesday, it announced new arrangements around Christmas.
It initially said it was only advising people to restrict gatherings to two households – plus an additional single person household. Later it said this was going to be made a legal requirement.
Drakeford insisted the central message was the same, whether it was law or advice, and suggested it was up to individuals to follow the rules: “Self-policing is the way this has to work,” he said on Thursday.
The Conservative shadow health minister, Andrew RT Davies, said the data issue undermined public confidence.
He said: “This is another staggering data cock-up during the health minister’s [Vaughan Gething] management of the Covid crisis, to add to the shielding letters sent in error and the data breach in September that saw 18,000-odd positive test results put on a public PHW server.”
Davies added that medical professionals had contacted him to voice their concerns as to whether the planned maintenance should have been conducted when it was.
Davies continued: “Welsh Labour and their failing health minister have clearly lost control of the virus in Wales. This loss of control can only be compounded by today’s news of yet another data mix up. An investigation into this fiasco should be launched.”
PHW said the problem was related to the maintenance of the NHS Welsh Laboratory Information Management System.
A spokesperson said: “This took place on Friday and Saturday (11-12 December) so that essential service upgrades could take place. This means that we expect a backlog of results to come through over the coming days, and there will be a period of data reconciliation and validation that will affect our daily reporting figures for several days.
“[Thursday’s] figures and those over the following days will be higher as a result of the backlog coming through. Across Wales, [on Thursday] we will be reporting in the region of 11,000 new positive cases. Full breakdown by local regions is currently being analysed and will be available in [Thursday’s] dashboard update.
“We stress this has not affected individuals receiving their results, and anyone who tests positive will be contacted by their local authority’s test, trace, protect team in the usual way. Interim data was shared with stakeholders.”
The Plaid Cymru health spokesman, Rhun ap Iorwerth, told the BBC: “With positive cases in Wales rising to record levels, it is crucial that the reporting of data is both timely and robust.
“The public need a complete and current picture of the situation to realise the gravity of what we are facing. We need urgent reassurance that the failings have been addressed.”