In his resignation letter to Ed Miliband, Tom Watson told the party leader to "be the great Labour leader that you can be".
The letter reads: "John Humphrys asked me why you were not at Glastonbury this weekend. I said Labour leaders can’t be seen standing in muddy fields listening to bands.
"And then I thought how terribly sad that this is true. So: be that great Labour leader that you can be, but try to have a real life too. And if you want to see an awesome band, I recommend Drenge."
Labour leader Ed Miliband has turned on the trade union Unite - his biggest financial backer - whose votes helped him become leader.
Tom Watson has announced his resignation from the shadow cabinet, amid a row about the influence of the union Unite on the Labour party.