David Letterman interrupts Steve Martin and Martin Short's live performance in Texas to add his commentary to Donald Trump's presidential campaign.

David Letterman has revealed he may have pulled the pin too early on the Late Show.

With tongue firmly in cheek, the recently retired comic said quitting just before Donald Trump announced his run at the presidency was "the biggest mistake of my life".

"I had no regrets. None. I was happy... And then a couple of days ago, Donald Trump said he was running for president," Letterman said in a surprise appearance during a stage show by his mates Martin Short and Steve Martin. "I have made the biggest mistake of my life."

Retired, not retiring ... David Letterman has made a surprise appearance on stage with Steve Martin and Martin Short.

With that, Letterman pulled one of his famous top ten lists from his pocket and launched into "Ten Interesting Facts About Donald Trump".

Among the hitherto unrevealed facts about the widely lampooned candidate are that the "thing on his head" was the gopher in Caddyshack and that he "calls out his own name during sex".

The number 10 Trump "fact" was: "Thanks to Donald Trump, the Republican mascot is also an ass."

Democrats have hailed Trump's controversial candidacy as a huge boost to their cause.