The Cabinet Office has rejected Dominic Cummings' claim that China breached high-level systems used to transfer sensitive government information.
In an interview with the Times, Cummings said China obtained "vast amounts" of "extremely secret" information from the UK intelligence services and parts of Whitehall.
He told the paper the breach was covered up after he was briefed on the compromised data in 2020 while a senior aide to then-Prime Minister Boris Johnson.
In response, a Cabinet Office spokesperson said: "It is untrue to claim that the systems we use to transfer the most sensitive government information have been compromised."