Blake Shelton performs birthday duet with Gwen Stefani

Blake Shelton rang in his 40th birthday onstage on Saturday (18Jun16) as his girlfriend Gwen Stefani stunned fans with a surprise performance.

The country star celebrated the milestone at the Country Jam festival in Grand Junction, Colorado, where he was joined by the No Doubt frontwoman onstage.

At the end of Blake's two-hour set, Gwen appeared unannounced to join her beau for their duet Go Ahead and Break My Heart, much to the crowd's delight.

Blake then told the audience, "This is fun! Y'all have been so freakin' nice to me on my 40th birthday and I guess if I had one birthday wish for me personally... I wish I could rock. I wish I was one of those type of artists that could rock but I can't."

Clearly egging Gwen on, he continued, "For my birthday I wish I could hear an artist who could rock, that's my birthday wish."

The singer happily obliged and she performed her band's hit song Hella Good, urging fans to jump up and down to celebrate Blake's birthday.

Meanwhile, Gwen, 46, shared the offstage celebrations on Snapchat, posting videos and photos from the decorated private plane they travelled in, including one snap featuring Blake wearing tinted "Happy Birthday" glasses and a 40th birthday badge.

The singer showed off one of Blake's gifts - a pair of wine glasses with caricatures of Blake and Gwen, dressed in the same outfits from the Vanity Fair's Oscars after party earlier this year (16), when the couple made its red carpet debut.

Gwen also posted shots of Blake's two cakes, one shaped like a black acoustic guitar and another a classic chocolate cake from his band and road crew.