Shawn Mendes and Miley Cyrus. Picture: Instagram

Shawn Mendes and Miley Cyrus are reportedly working on a secret duet which they are believed to be performing at the Grammys on Sunday.

The 20-year-old Canadian pop star and the 'Nothing Breaks Like a Heart' hitmaker are both set to perform at the Grammys on February 10, and the latter got their fans guessing they could be singing a duet together at the ceremony, after Miley shared a picture of herself rehearsing in front of some of Shawn's instruments.

Now, TMZ reports that the pair are in fact working on a secret song together. 

Eagle-eyed fans of Shawn - who is nominated for Pop Album of The Year for his self-titled third studio album and Song of The Year for hit single 'In My Blood' - noticed that his name was written on one of the guitars.

Miley, 26, had captioned the Instagram post: "Gram Gram rehearsals! Performing this Sunday at the @recordingacademy ! (sic)"

A fan of Miley also posted a picture of both artists rehearsing separately on the app and mentioned the secret duet, and the pop star commented with a love heart emoji, adding fuel to the speculation. 

The user with the handle @romcyrus wrote: @MileyCyrus is rumored to be performing a surprise song with @ShawnMendes at the @recordingacademy after liking some IG posts and posing in front of Shawn's guitars.

Credit: @cyrusreports 

I'M SO FUCKING READY @mileycyrus @shawnmendes (sic)"

Miley recently revealed she has reunited with her 'Bangerz' producer Mike WiLL Made-It on her follow-up to 2017's 'Younger Now', as well as recent collaborator Mark Ronson and Andrew Wyatt.

Asked what fans can expect, she said recently: "A song that [Ronson] and I have done together is more rock driven, modern Debbie Harry or Joan Jett.

"Then we've got songs with Mike WiLL that lean more hip-hop and songs with Andrew Wyatt that lean more pop/alternative. I just have kind of everything."

The 'Malibu' hitmaker - who is marred to Liam Hemsworth - started work on her next record before her last album was released, in September 2017, and promised to release some of the songs this year.

She spilled: "I don't have any definite dates, but definitely in the new year you guys are gonna be hearing a lot more music."

This could mean that the new track with Shawn will feature on her new record.