As he boarded the night train to Ukraine, Boris Johnson had the usual entourage of aides and bodyguards – plus the man who had given him £1m.

Less than a year had passed since Johnson accepted what is thought to be the largest donation ever to an individual MP. It was from Christopher Harborne, one of the UK’s biggest and most private political donors.

Harborne, whose millions helped bankroll Brexit, made the payment to a private company Johnson set up after resigning as prime minister. Now leaked files show that Johnson, a champion of Ukraine in office and since, was accompanied in September 2023 by his benefactor on a two-day visit that included meetings with top officials.