Key Facts

• John Healey has resigned as defence secretary, a move that is being described as a disaster for the prime minister.
• The article is written by Laura Kuenssberg, an expert news analyst.
• The resignation is taking place in the context of Downing Street, which suggests that the UK government is experiencing turmoil.
• No specific reason for John Healey's resignation is provided in the given article snippet.
• The article suggests that the resignation may lead to a ripple effect, with the phrase "Downing Street dominoes about to fall" implying that further changes or crises may be imminent.