Jane Land and Matthew Glove are the founders of Veganuary and their latest project, Million Dollar Vegan, has received backing from a number of celebrities including Paul McCartney and Manchester United footballer Chris Smalling.
The face of the campaign is twelve-year-old American animal rights and environmental campaigner Genesis Butler.
The letter, signed by the youngster to the pontiff, reads: "As we approach the period of Lent - a season of reflection and hope - I write to you today with the utmost respect and appreciation for your speaking out on climate change, habitat loss and pollution, and for reminding the world that Earth is a home we all share.
"In your encyclical letter, Laudato si', you stated that every effort to protect and improve our world will involve changes in lifestyle, production and consumption. I agree with all my heart and seek your support in tackling one of the largest underlying causes of the problems we face: animal agriculture.
"Your Holiness, we must act. Moving towards a plant-based diet will have substantial environmental benefits. It will protect our land, trees, oceans and air, and help feed the world's most vulnerable.
Nearly 30,000 people have signed a change.org petition calling for the pope to accept the challenge and go vegan from 6th March - 18th April.
The Vatican is yet to respond.
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