In a video posted on social media, Popp said: "I have always stressed that my gut will make the decision, and now it has. After long, tearful deliberations, I have decided with a heavy heart to end my career with the national team. The fire that ignited in me 18 years ago and grew stronger from year to year has now almost burnt out.
"It was always important to me to make this decisive decision myself, from within myself. Neither my body, which is a ticking time bomb, nor another person should get ahead of me. Before the fire is completely extinguished - because then it would be too late - the right time has now come."