Avril Lavigne and Chad Kroeger have announced their engagement after a secret six-month relationship.

The "Sk8er Boi" singer reportedly fell for the Nickelback singer in February, when the two Canadian stars joined up to write a song together for her new album.

One of Avril's pals tells People magazine, "A romantic relationship blossomed as they spent time writing together."

The couple were able to keep their relationship very private and their news has shocked fans around the world.

Just hours ago the news was posted on Nickelback's twitter page that Kroeger, 37, had proposed to Lavigne, 27; and she re-tweeted the message shortly after.

Avril was previously married to Sum 41 singer Deryck Whibley; and she split from reality TV star Brody Jenner early in 2012.

As for the single the couple were working on, which will appear on Avril's upcoming fifth album; well, they finished that up on Sunday.

"That's a wrap boyz @hodgesmusic and chad @Nickelback !!! Woo hoo. Super stoked for this record," Avril tweeted.