Anti-bullying campaigner Natalie Harvey has taken to social media to express her offence at the promotional poster, which appears on the side of a grocery store on Tamworth Road, Nottinghamshire.
The six-metre-high billboard features aliens and a UFO with the caption; 'They're coming and when they arrive they'll take the fat ones first!'
Natalie, who set up the charity Combat Bullying and first saw the poster last Tuesday, said; 'Just this week alone I've had three cases of bullying due to weight issues and I feel like campaigns like this aid bullying.
'I couldn't believe it when I saw it. It's 2016, this sort of fat-shaming humour is offensive... we have a responsibility to say something.'
The government campaigner worries that 'children are so fragile' and the 'playground humour' used in this advertising could cause further harm.
She added; 'It just doesn't sit well with me... If those children or the perpetrators saw this poster it would cause further harm for the children who are being bullied.'
However, Natalie's call for the poster to be taken down has been met with refusal by the gym CEO, Jan Spaticchia.
Jan has refused to take action, despite the shop the poster appears on also requesting it be removed from their wall.
Jan said; 'The aliens campaign is actually very successful. We aim to get people talking and promote the notion of a healthy lifestyle.
'We certainly didn't mean to cause offence and we care about the relationships we build with the communities we serve.
The CEO also offered to donate £250 to Natalie Harvey's anti-bullying charity live on air whilst doing an interview on Capital FM. Natalie refused the offer.
'We have found that by taking a light-hearted approach we can connect with more people who would enjoy what we have to offer. It's a little harmless fun.'
After the refusal to remove the poster, Natalie has since tried to rally celebrity support, tweeting Prison Break actor Wentworth Miller after he recently admitted that he struggled with depression-related weight-gain.
She tweeted the star; 'I am campaigning for Advertising Standards Agency to remove this ad could i ask your opinion as I'm getting a lot of abuse.' GoodtoKnow have contacted Fit4less for comment.