There is a small but deeply passionate group of people convinced of the "fake Melania" conspiracy theory.

Just as flat-earthers deny the planet is round, and some people insist climate change is a hoax, those who subscribe to the 'fake Melania' theory believe America's First Lady has a body-double who occasionally stands in for Melania Trump.

On Thursday President Trump hit back at the rumours in a furious post on Twitter, even going as far as to claim the "fake news media" doctors the images to fuel the theory, reports news.com.au.

"The Fake News photoshopped pictures of Melania, then propelled conspiracy theories that it's actually not her by my side in Alabama and other places," the president tweeted.

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"They are only getting more deranged with time."

Twitter users had some clever retorts to the tweet.

"Photoshopped??? Like your inauguration crowd size?" one replied.

"And this is what you are worrying about during a 'National Emergency?'" another asked.

Actress Mia Farrow queried "Between your impressively exhaustive TV watching, and tweeting, when do you actually work?"

Chatter grew on Twitter last week when the President and First Lady visited Alabama to tour the devastation left by a tornado that killed 23 people.

During the Alabama trip, the Trumps paid a sombre visit to a victims' memorial — a public outing covered by WSFA 12 News reporter Sally Pitts.

But when Ms Pitts shared a picture of the couple on Twitter, it didn't take long for social media users to spot something unusual about Mrs Trump.

Some claimed the woman next to the president was just not Mrs Trump, pointing out discrepancies between "fake Melania" and the "real" Mrs Trump's appearance.

The tweet has received thousands of likes, retweets and comments, with countless people pointing out the woman in the photo appears to have a different chin, height, hair part and cheek structure to the First Lady.

Many noted the mother-of-one's body language appeared to be more open and relaxed than normal during her Alabama visit and her posture and even the way she walked were slightly off, adding more fuel to the fire.

And many argued the couple being photographed holding hands was further proof of a body double, with the 48-year-old famously evading her husband's attempts to hold her hand at many public events in the past.

In July 2018, the bizarre theory went into overdrive after the President and his wife were pictured stepping off air force One in Brussels ahead of a NATO summit.

In those photos, as well as others taken during the Brussels jaunt, Mrs Trump also looked drastically different.

That appearance came just months after Mrs Trump mysteriously disappeared from public view for several months after White House staff announced she was being treated for a kidney condition.

Her absence sparked endless rumours at the time, with some believing Mrs Trump's illness was used as an excuse to cover up her moving back to Trump Tower in New York with 12-year-old son Barron or undergoing a facelift.

Many who believe in the theory are convinced Mrs Trump's body double is a secret service agent who has an uncanny resemblance to the First Lady — and the rumours have been fuelled further by Melania Trump's penchant for sunglasses, clothing and accessories that often cover part of her face.