The former Spice Girls singer feared she would die after being stricken by eating disorder bulimia nervosa - which sees sufferers binge eat before purging themselves of food - and only sought help for her problems after the 'Angels' hitmaker encouraged her to book into a US rehabilitation clinic.
She said: "I was worried I'd get fat. I would binge and then felt fatter and would make myself sick. It was awful. Robbie knew about my bulimia and he advised me to get help. He told me to go into rehab and that possibly saved my life. The bulimia would have got worse without it. I will always be grateful to him, always."
The 37-year-old British star - who quit the Spice Girls in 1998 - says she bonded with Robbie because he understood her situation, having struggled with his own problems when he left Take That in 1995.
Speaking on British TV show 'Piers Morgan's Life Stories', she explained: "I became really close to Robbie after leaving the Spice Girls and it was a very poignant friendship. I was lonely and felt he was the only person on the planet who could understand me because of his experiences with Take That."
Geri, who was romantically linked with Robbie back in 2000, also claimed they were more like really good friends than lovers.
She said: "We understood each other. But I didn't really go out with him."