US pop star Meghan Trainor has responded to her image being used in a campaign against same-sex marriage, insisting the use of the photo was not authorised.

As ABC reports, the now-deleted photo, posted on the Vote NO Australia Facebook page, showed Trainor’s face alongside the caption, "My vote is NO, you need to let it go", in reference to her 2016 track, NO.

After being notified about the advertisement, Trainor took to Twitter to distance herself from the campaign.

"SUPPORT MARRIAGE EQUALITY! Someone in Australia is illegally using my picture for a campaign against marriage equality. So wrong. Not okay."

The Vote No Australia page has since issued an apology for using the unauthorised image of Trainor on its Facebook.

"It has come to our attention that a meme was posted on the page by a new editor that was not correctly vetted of an opinion of a celebrity," the statement reads.

"Once it came to our attention it was immediately removed. We apologise to Meghan Trainor and her support base if this has caused any angst or distress. We had no idea who the person in the meme was."

See the full statement below.