The Prime Minister has accepted she used non-comparable figures in criticising hospital waiting times in Wales.
It comes as the head of the UK Statistics Authority agreed with First Minister Carwyn Jones that Mrs May misused official figures during Prime Minister's Questions.
Last month Mr Jones wrote to Sir David Norgrove, who chairs the authority, after Mrs May claimed that waiting times in accident and emergency units in Wales were on average much longer in Wales than in England.
Mr Jones later accused the Prime Minister of not comparing like with like.
When challenged by Labour leader Jeremy Corbyn about the NHS, Mrs May said the latest figures showed 497 people waiting more than 12 hours in A&E at hospitals in England compared to 3,741 in Wales.
In a letter to Sir David Norgrove, Mr Jones said it was “simply not a valid comparison”.
Mrs May since responded with a clarification but added the figures she should have used still reflect badly on Wales.