Durham Police have formally been asked to "establish the facts" around Dominic Cummings' visit - including whether he broke the law.
Police Commissioner Steve White today wrote to the force's Chief Constable, Jo Farrell, asking her to see if any rules were broken when Mr Cummings visited County Durham at the end of March.
Retired teacher Robin Lees, 71, reported Mr Cummings to police yesterday after insisting he saw the top aide out in Barnard Castle - 30 miles from where he had stayed for two weeks - on April 12.
A witness also told the Sunday Mirror they saw Mr Cummings walking in County Durham on April 19, days he returned to work in London.
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