The letter commits each signatory to disclose, while respecting client confidentiality, the percentage of their turnover categorised by industry, including income from fossil fuel companies and other high-carbon clients by the end of this year.
Several agencies have already disclosed the information.
The letter reads:
"We know many of our colleagues and friends across the creative industry are anxious/terrified about the climate emergency. We also know that disclosing climate conflicts will be too early, and too controversial, for many agencies today.
"But, we firmly believe that we cannot serve climate solutions, whilst still serving the industry’s most answerable for causing the climate emergency.
"Disclosure is only the first step on a journey that must lead to divestment - divesting agency client rosters of these clients. Agencies need to align our businesses with the climate science, just like everyone else.
"Thanks for the nudge, XR."
Futerra co-founder Solitaire Townsend said the agency spearheaded the letter to "help put agencies on the right side of history".
She added: "Creativity, invention and storytelling will help solve the climate emergency. But not if the advertising, PR and marketing industry keep serving the problem. Declaring your agency purpose without disclosing proof simply won’t cut it anymore."
William Skeaping, an activist and member of Extinction Rebellion with a background in advertising and branding, said it was a first step towards the group's first demand – of governments and business to 'Tell The Truth'.
He added: "Industries need to come together, be open, honest, disclose their business actions that are in conflict with life itself."
Last month, the climate group staged a sit-in at Cannes Lions, and in May they sent an open letter urging the advertising industry to act on climate destruction.
Skeaping also called on agency employees to demand change: "So many agencies claim to understand Gen-Z audiences yet promote ecologically damaging products and behaviours - even greenwash fossil fuel companies - will ensure that these young people who are deeply concerned for their planet, will not have a future at all."
Signatories so far (*client disclosure report already available):
Futerra* - Solitaire Townsend, Lucy Shea
Greenhouse PR* - Anna Guyer
Barley Communications – Sam Williams
Question and Retain – Annabel Dunstan
L & Co – Paul Barlow
Forster* - Amanda Powell-Smith
Leap – Matt Hocking
Studio Republic – Chris Todhunter
Good Collection – Vicki Nice
Creative Concern – Steve Connor
Don't Cry Wolf – John Brown
Praytell – Andy Pray
Revolt – Alex Lewis and Peter Bardell
Republic of Everyone* - Ben Peacock
Reason Digital – Matt Haworth
Croud – Luke Smith
Mellor & Smith – Paul Mellor
The full letter, sign up for agencies and individual creatives, agency client disclosure reports, and other resources are available here.