Boris Johnson allegedly blamed high rates of Covid-19 in Wales on “singing and obesity”.
The apparent comments were included in the diary of Sir Patrick Vallance, the Government’s former chief scientific adviser.
They were dated September 2020.
Sir Patrick wrote: “Wales very high – PM says ‘It is the singing and the obesity… I never said that’.”
The full context of the alleged comments is unclear.
Mr Johnson was presented with the diary extract by Pete Weatherby KC, who represents Covid Bereaved Families for Justice, at the UK Covid-19 Inquiry on Thursday, but he was not asked about the entry.
Parts of Wales in September 2020 were among the areas of the UK hardest hit by the virus.
Earlier in the day Johnson became teary as he recalled his own experience with the virus.
He said: “When I went into intensive care, I saw around me a lot of people… And some of us were going to make it and some of us weren’t.”
Watch the clip in full below: