American singer Gwen Stefani made a surprise appearance during a video interview that her boyfriend, country singer Blake Shelton, was having with The Tonight Show host Jimmy Fallon.

The couple have been self-isolating in Shelton's house in Oklahoma.

Around 2½ minutes into the interview earlier this week, Shelton, 43, said it had disrupted a schedule that he and Stefani, 50, had lined up for the day. The schedule referred to a haircut that Stefani was going to give him.

Stefani then stepped into the frame holding an electric shaver, prompting applause from an excited-looking Fallon who requested that she give Shelton a mullet. And that was exactly what she did.

At one point, Fallon asked Shelton to go closer to the camera so he could judge the haircut, and told Stefani it was not long enough at the back, but it was "getting close".

Fallon then spoke about MusiCares, a non-profit that supports musicians. He invited people to donate to a relief fund set up by the organisation for people in the music industry who have been affected by the Covid-19 pandemic.

The video ended with Stefani shaving the initials "J" and " F" - for Jimmy Fallon - into Shelton's freshly shorn head.

The couple, who are both judges on the television singing competition The Voice, have been publicly dating since 2015.