The fashion designer's spokeswoman said: "It is highly inappropriate to trivialise such a disorder, and defamatory to be so thoughtless with a person's reputation in this way.
"Sadly this is now a legal matter."
The takeaway restaurant came under fire for the advert yesterday after an anorexia charity claimed that the shocking slogan "puts people at genuine risk" of eating disorders.
However, manager Soni Sidhu said that he never meant to offend anyone with the advert, which has been on the back of the van for about three years - and that his customers see it in context.
He said: "As the manger and on the behalf of all our staff and owners I would like to state we recognise how serious eating disorders are and would never make light the seriousness of people with eating disorders.
"We would like our customers and all people in general to take our advertising in context. We are not a fly-by-night business trying to make a quick buck.
"Anorexia and any mental illness are very serious."
The 32-year-old said that the company was "horrified and hurt" by comments made by Marg Oaten MBE, of the Seed anorexia charity yesterday, who said that the advert would promote eating disorders.
However, he said that his customers were sensible enough to realise that the advert was not meant to offend.
Soni added: "We have always stated even if one individual is upset or offended by our advert we would be more than happy to take it down.
"From all the comments on newspaper websites and our Facebook page we have received nothing but support.
"It is reassuring the general public are sensible enough to differentiate between make-believe and real life.
"It is offered as a fun way to make people smile, and to escape from the daily hustle and bustle of life. We would be genuinely horrified if anyone was genuinely offended.
"If, in 2017 Britain, we are asked to take down this advert it will be a sad day for freedom of expression."
Marg, who has an MBE for her services to help those battling eating disorders, had said yesterday: "To trivialise anorexia in a pizza advert is appalling.
"There has been a lot of good work done around mental health recently, especially by the royals, in reducing the stigma attached to eating disorders.
"This is a step in the wrong direction. The people responsible for this should hang their heads in shame.
"The advert puts people at genuine risk. Those who suffer from eating disorders are constantly battling with their feelings and thoughts.
"They will see the advert and start comparing themselves to the size of Victoria Beckham.
"These people need to be brought to account. There should be legislation banning this kind of thing."
