'Young' is the fastest selling debut single of the year so far, and the second fastest overall behind DJ Fresh's 'Hot Right Now'.
The former N-Dubz star ended Carly Rae Jepsen's four-week reign at the top with 'Call Me Maybe', which falls to two.
fun. have risen to third place with their huge US hit 'We Are Young', while Cover Drive score another top five smash with 'Sparks' at four. Alex Clare completes the top five with 'Too Close'.
Train score their highest ever UK chart placing, as 'Drive By' climbs a further three places to seven. Calvin Harris and Ne-Yo drop four spots to six with 'Let's Go'.
Jessie J drops three to eight with 'LaserLight', with Nicki Minaj and Gotye's long-runners 'Starships' and 'Somebody That I Used to Know' completing the Top 10.
Marina and the Diamonds almost achieved her first Top 10 hit with 'Primadonna' climbing back up two at 11, while Katy Perry's 'Part of Me' leaps eight spots to 16.
Rihanna's latest single 'Where Have You Been' entered the Top 40 for the first time at 21, while Sub Focus achieved a new entry at 23 with 'Out the Blue'.
Dance act Avicii landed at 26 with new single 'Silhouettes', with 'Daydreamer' by Flux Pavilion and Example squeezed in at 39.
Train's rediscovered popularity saw their 2010 hit 'Hey, Soul Sister' re-enter at 38, while James Vincent McMorrow's cover of Steve Winwood's 'Higher Love' climbs up to 37 after its use on The Voice UK.
Source: Official Charts Company
> This week's album chart
Watch the video for Tulisa's 'Young' below: