The Spice Girls star has heaped praise on the 18-year-old singer - who has used her platform to encourage fans with body positivity and to speak out about racial injustice - and she insisted she doing things ''in the best way''.
She told 'Entertainment Tonight': ''She's so wise beyond her years. She knows she has a very powerful platform to be speaking from and she's using it in the best way.
''And, it's brave because everybody has an opinion and everybody is ready to criticise when you're in those positions. I feel extremely proud of her.''
Meanwhile, the 'Wannabe' hitmaker admitted things are tougher now for ''all younger artists'' because they have to cope with the pressures of social media.
She added: ''I feel for all younger artists now because the Spice Girls didn't have to deal with social media.
''It was a very, very different time. We felt pressure [and] we felt like we had a responsibility to our younger fans, but now you are literally under the spotlight.
''She handles it beautifully, so more power to her.''
She and Billie have become friends, and Mel was moved when she saw the 'Bad Guy' star perform an intimate theatre show in London late last year.
She said: ''I went along to see her show in London towards the end of last year - she was playing quite a small theatre in London. It was 2,000 people and she'd already outgrown it.
''She's such a huge star, but it was one of those moments when you think, 'Wow, being in this audience is such a privilege,' because you're never going to see her in this environment again.'
''The room was full predominately of teenage girls, screaming and singing every word, every ad lib, and I just felt this really mad connection because it took me back to being a Spice Girl.''
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