Alan Johnson has quit as shadow chancellor over a crisis in his personal life.
Mr Johnson announced that he was standing down for "personal reasons" amid reports that his second marriage was in trouble.
He and colleagues refused to elaborate as the Westminster rumour mill went into overdrive.
Shadow minister Tessa Jowell told BBC2's Newsnight she did not know why Mr Johnson had resigned, but added: "He's clearly very distressed. It's clearly a very, very difficult time for him."
In an earlier statement, Mr Johnson said only that it was "to do with my family", adding: "I have found it difficult to cope with these personal issues in my private life whilst carrying out an important front bench role."
Mr Johnson has been married to his second wife Laura for almost 20 years and the couple have a 10-year-old son.
His resignation left Ed Miliband facing his biggest test since being elected Labour leader, forcing him to reshuffle his shadow cabinet less than four months into the job.
Mr Miliband installed Ed Balls as shadow chancellor after overlooking him last October. Amid suggestions that Mr Balls may drag Labour to the left, Mr Miliband stressed that there would be no change in economic policy.
But the Tories and Liberal Democrats said Mr Balls' appointment marked a return to strength for Mr Brown's old guard. Conservative Party deputy chairman Michael Fallon said: "It beggars belief that Ed Balls has been appointed as shadow chancellor of the Exchequer.
"The man who is responsible for Britain's economic mess has returned. The Labour Party has learnt nothing and is now led entirely by Gordon Brown's old team."