It followed a complaint from the Real Bread Campaign, run by the food and farming charity Sustain, which has scrutinised the sandwich chain for a number of years.
Information on Pret's Facebook page and website claim the firm makes "good natural food", "doing the right thing... naturally" and has a "mission" of "avoiding the obscure chemicals, additives and preservatives common to so much of the 'prepared' and 'fast' food on the market today". The campaign said this implied no artificial additives were used in Pret foods.
Responding to the ASA, Pret said that the word "mission" suggested that additive and preservative-free food would be "an ideal state or their ultimate goal", rather than its current state. Pret also said the mission statement's wording had been cleared by Westminster Trading Standards in 2011.
The ASA noted that food containing E-numbers could not be considered natural as per Food Standards Agency guidance, and banned the company from producing material calling its food "natural".
Time to 'walk the walk'
Real Bread Campaign coordinator Chris Young said: "We welcome this ban, which sets a precedent that sends a clear message to food companies that unless they walk the natural food walk, it’s misleading to talk the natural food talk.
"This ruling is good for shoppers whom the ASA is there to protect, and for the small, independent bakery and eatery owner who serve genuinely all-natural Real Bread sandwiches."
However, a second complaint from Sustain about a Flickr post on Pret's in-store ovens was rejected.
Pret said on Flickr that it bakes a range of products "in-house throughout the day" - Sustain claimed this was misleading as products are made off-site and heated in store, but the ASA rejected this, saying consumers were "unlikely to interpret 'baked in store' to mean only products that were made from scratch".
Pret's food and coffee director Clare Clough said: "Since day one, Pret has been on a mission to create handmade, natural food, avoiding obscure chemicals and preservatives. Our teams freshly prepare our food every day in our on-site kitchens and at the end of each day our unsold food is donated to homeless charities.
"We do, of course, take on board the views of the ASA and have already made the requested changes. We cherish the relationship we have with our customers. We believe we represent Pret’s food honestly and we always welcome feedback."
The company also told the ASA it was committed to auditing all marketing material.
Separately, the Real Bread Campaign said it was continuing a campaign about similar claims made by Pret on posters, wall boards, napkins, stickers, product packaging, window displays and other marketing material in its outlets.
These are not covered by the ASA's remit, and the campaign is pursuing Westminster City Council in its role as the primary authority for Pret.