British actress Emily Blunt was "astonished" by the backlash she received after making a joke about her U.S. citizenship.

The Devil Wears Prada star was sworn in as an American in August (15) and she later made a gag about her new status in a TV interview.

During an appearance on talk show Jimmy Kimmel Live!, she joked about a political debate between Republican presidential hopefuls, including Donald Trump.

"I became an American citizen recently," said Emily. "And that night, we watched the Republican debate and I thought, 'This was a terrible mistake. What have I done?'"

The comment sparked outrage online and among right-wing pundits, who heavily criticised the Brit over the gag.

She subsequently issued an apology, saying, "It was so not the intention to hurt anybody or cause any offence so I really apologise to those that I caused offence (to). It was just an offhand joke... I think I'll probably leave the political jokes to (talk show) Late Night or something."

Emily has now opened up about the controversy, revealing she was shocked by the vitriol aimed at her over what she thought was a harmless joke.

"It was a joke!... I was bummed," she told shock jock Howard Stern on his SiriusXM radio show.

"I was sort of astonished by the outrage, because it was meant completely as an innocuous joke. So I think I was more just quite taken aback, so it did bum me out... It was like catnip (for critics)... I think it does make you feel like you should be a little cagier..."

Emily, who is married to American comedian John Krasinski, goes on to reveal she passed her citizenship test with flying colours: "It was hard. I did (study)... You have to get six out of 10 (questions) right, and I got the first six right... so they stopped... I do (feel good about being a U.S. citizen). I have a real affection for the country."