James Cordon and Gwen Stefani sing it up – but then these two mega-stars jump in the back seat!

James Cordon’s Carpool Karaoke has been a big hit with viewers for a number of years now, but the entertaining slot looks like its ramping up a gear!

Regular watchers of the segment will know it features James in a car driving to “work.” He’s regularly joined by some amazing singing talent, from award-winning solo artists to stadium rock bands – James as had everyone in the back of his car!

This time, it’s Gwen Stefani’s turn. The gorgeous lead singer of No Doubt is on top form as the pair rattle through her biggest hits, James in the driving seat – doing his very best to get those high notes! In between numbers, James always has a candid interview with the star of the day, and it’s fascinating to watch and listen to his rapport with Gwen on this occasion. From the dashboard cam, the two throw some serious shapes while singing – as there’s always extra kudos for enthusiastic car seat dancing! But the star power doesn’t stop there – there’s a huge treat in store for the watching fans!

Apparently, there must be four people in the car in order for them to be allowed to use the carpool lane. James does a brilliant job of claiming that he’s going to be late for work if he can’t get through – and then pulls over to call some backup! James tells Gwen that the guy he’s calling is never doing anything anyway – and then viewers are astonished when non-other than George Clooney climbs into the back seat! And the crowd goes wild!

But of course, three people isn’t good enough! Four are needed in order to use the lane and make sure that James gets to work on time, and this time it’s George’s turn to call in a favour. Seconds later, Hollywood legend Julie Roberts has joined them! And the crowd goes wild again!

With the four stars all buckled up, it’s time for the tunes – after all that’s what we’re here for! If it doesn’t put a smile on your face when all four celebs bust out a rendition of Hollaback Girl, you’re not human! It’s hilarious to watch them all get into the car seat dancing, and it must have been a crazy sight to see if you happen to pull up next to them at a stop light! George, in particular, looks like he’s having the time of his life! Check out the video below for one of the best ever Carpool Karaokes!