Tonight Show host Jimmy Fallon has apologised after a historic clip of him wearing blackface on Saturday Night Live went viral on Monday.

The 45-year-old insisted he was ’very sorry’ about the decision to do the performance and thanked those who brought it to the public’s attention.

The sketch, which was from an edition of the show 20 years ago, saw Fallon impersonate African-American comedian Chris Rock.

As the skit gathered pace on social media on Monday and criticism of the TV presenter began pouring in, he decided to speak out.

Taking to Twitter, he said: "In 2000, while on SNL, I made a terrible decision to do an impersonation of Chris Rock while in blackface. There is no excuse for this.

“I am very sorry for making this unquestionably offensive decision and thank all of you for holding me accountable."

Rock, who is a critically acclaimed comedian, is yet to publicly speak out about the sketch.

Fallon has hosted the Tonight Show for well over 1000 episodes since taking over from the legendary Jay Leno in 2014.