By Jonathan Isaby
Apparently it is for "personal reasons", whatever that may entail.
Sky News is reporting that Ed Balls is to take the shadow Chancellor portfolio.
More when we have it.
Ed Balls is confirmed as Shadow Chancellor; his wife Yvette Cooper takes his job as shadow home secretary; Douglas Alexander takes her former portfolio of shadow foreign secretary; Lima Byrne becomes shadow work and pensions secretary; Tessa Jowell to take on responsibilities shadowing the Cabinet Office, formerly held by Byrne.
Johnson statement confirms the resignation is for personal reasons relating to his family: "I've found it difficult to cope with these personal issues in my private life whilst carrying out an important front bench role."