Cancer Research UK has defended its latest obesity campaign after a group of scientists and healthcare professionals called on the charity to put a stop to the "harmful and misleading" adverts.
The group said CRUK's fundraising partnership with Slimming World, which has generated more than £13m since it began in 2013, has a "clear conflict of interest" and "cannot be justified".
CRUK launched a national campaign this week to increase awareness of the link between obesity and cancer.
The charity published figures saying excess weight causes more cases of certain cancers than smoking and urged the government to do more to tackle obesity.
This prompted the group to create the petition, which had generated more than 800 signatures on Friday afternoon, and send an open letter to Michelle Mitchell, chief executive of CRUK.
The letter says: "Implying that individuals are largely in control of and responsible for their body size (and therefore cancer) supports a culture of blame and plays into prejudices and negative stereotypes, which drive the social exclusion, marginalisation and inequality of an already stigmatised population.
"The evidence is clear that weight stigma actively discourages people from engaging in behaviours that are known to promote health and reduce cancer risk."
The group recommends CRUK prioritises wellbeing over weight by delivering "weight-inclusive" messages rather than focusing negatively on obesity.
Laura Thomas, director of the London Centre for Intuitive Eating and a signatory of the letter, told Third Sector the group was formed by people worried about CRUK's failure to heed concerns raised last year about its obesity messages.
"We would like to see CRUK halt its current campaign and make efforts to change tactics," she said.
"It is absolutely terrifying that a cancer charity might inadvertently be causing people not to engage with public health initiatives."
The letter also claims there is a "clear conflict of interest" in CRUK's partnership with Slimming World.
It says: "The Slimming World brand is ultimately committed to commercial gain, not improving public health.
"This partnership cannot be justified on scientific grounds as evidence demonstrates that these programmes are not effective ways of achieving and maintaining weight loss or preventing cancer.
"Therefore, as an evidence-based organisation, your association with Slimming World misleads your patrons and the public more broadly."
Sarah Woolnough, executive director of policy and information at CRUK, defended the obesity campaign.
"The link between obesity and cancer is well established and, based on the overall research evidence, we are confident that it is a causal link," she said.
"The World Cancer Research Fund International states this as a fact and the International Agency for Research on Cancer said in 2016 that there was strong evidence to show that extra body fat increased the risk of 13 different types of cancer.
"Our goal is to raise awareness of the link between obesity and cancer, while also calling on the government to make changes to our environment that will help make healthy options accessible for everyone."
Woolnough said the campaign was about national policy change rather than individual behaviour change and hadn't been influenced by any partners or funded by income from its Slimming World partnership.
"Slimming World does excellent work to support people who are overweight or obese in local communities across the UK, but our fundraising partnership with it is totally separate from this awareness raising and policy change campaign," she said.
"Over the past 50 years, smoking rates in the UK have dramatically declined thanks to public awareness of the health risks and policies such as banning tobacco advertising and increased taxation.
"We want to apply a similar approach to reduce the levels of obesity in the UK, including restrictions on the incessant advertising and price promotions for junk food that nudge us towards eating unhealthily."
A Slimming World spokeswoman said it was proud to support CRUK, but it hadn't "funded or influenced the decision to run this obesity campaign".
She added: "We’re strongly opposed to anything that stigmatises or shames overweight people. It is the opposite of Slimming World’s philosophy of respect and compassion."