Former England star David Beckham has come out for Remain.
The ex-Manchester United player said Europe is a team and it needs to play together to succeed.
"I played my best years at my boyhood club, Manchester United. I grew up with a core group of young British players that included Ryan Giggs, Paul Scholes, Nicky Butt and the Neville brothers. Added to that was an experienced group of older British players such as Gary Pallister, Steve Bruce and Paul Ince.
"Now that team might have gone on to win trophies but we were a better and more successful team because of a Danish goalkeeper, Peter Schmeichel, the leadership of an Irishman, Roy Keane, and the skill of a Frenchman in Eric Cantona.
"I was also privileged to play and live in Madrid, Milan and Paris with teammates from all around Europe and the world. Those great European cities and their passionate fans welcomed me and my family and gave us the opportunity to enjoy their unique and inspiring cultures and people.
"We live in a vibrant and connected world where together as a people we are strong. For our children and their children, we should be facing the problems of the world together and not alone.
"For these reasons I am voting to Remain," Beckham said.
Leading Leave campaigner Boris Johnson tried to play down the significance of Beckham's announcement, pointing out that the Brexit side too had backing from former England stars.
Challenged on LBC radio over whether the Beckham declaration was a blow, the former mayor of London came back: "Sol Campbell. John Barnes."
Prime Minister and Remain campaigner David Cameron welcomed Beckham's support, telling ITV's Lorraine show: "There was a very moving statement today from David Beckham talking about his children and saying how effectively, what he said to me was, 'You can't win in Europe, unless you're on the pitch'.
"And that's true. I worry about that as Prime Minister, I know that we sit around the table and make decisions about security and safety and fighting terrorism and all the rest of it, fighting climate change. And if we leave they don't stop meeting and making decisions that affect us, but they'll be making decisions about us but without us in the room and that would be bad for Britain."