Lorde has been named the hottest star under 21.

The 'Royals' hitmaker has topped Billboard magazine's annual ranking of ''music's most powerful minors'', with the 17-year-old star beating Australian pop band 5 Seconds of Summer and 'Believe' hitmaker Justin Bieber into second and third place respectively.

The publication hailed the group as ''perhaps the most impressive crop of under-21 stars'' they had ever assembled and tipped them all for bright futures.

They said: ''These 21 artists have already accomplished a great deal, but are safe bets to continue finding success as they edge toward adulthood.

''For now they're just kids, but they're the kids that everyone in music is watching.''

Rounding out the top five were Martin Garrix and Fifth Harmony, while further down the list included the likes of Chief Keef, Cody Simpson and The Vamps.

To mark the publication of the list, Billboard also compiled a chart of the 21 biggest hits from the past 21 years from artists who were under 21 at the time of release based on chart performances.

Topping the song list was Leann Rimes' hit 'How Do I Live', which was released in 1997 when the star was just 15 years old.

Mario's 'Let Me Love You' - released when he was 21 in 2005 - took second place, Lorde's 'Royals' came in third, Brandy & Monica's 'The Boy is Mine' was fourth and Usher's 'You Make Me Wanna' completed the top five.

Billboard magazine's 21 under 21 2014:

1. Lorde

2. 5 Seconds of Summer

3. Justin Bieber

4. Martin Garrix

5. Fifth Harmony

6. Austin Mahone

7. Earl Sweatshirt

8. Shawn Mendes

9. Becky G.

10. Chief Keef

11. Luis Coronel

12. Meghan Trainor

13. Cody Simpson

14. Madeon

15. Bobby Shmurda

16. Scotty McCreery

17. Troye Sivan

18. Rae Sremmurd

19. Akdong Musician

20. The Vamps

21. Unlocking The Truth