The appointment follows the creation of a press ad by the McCann creative partners, Brian Cooper and Jason Stewart, last month. The ad ran in The Independent and depicted two British Airways planes leaving vapour trails that resembled a "V-sign", and ran with the copy "BA's answer to climate change".
The Greenpeace director of communications, John Suaven, said: "I have become increasingly impressed by the team at McCann and their passion and commitment to Greenpeace. It therefore felt only right to formalise the relationship, and I am very excited to be working with them."
The McCann joint executive creative director, Simon Learman, said: "We are delighted to be working with Greenpeace. It offers us a fantastic creative opportunity for an incredibly important cause."
Greenpeace, which has previously produced the majority of its advertising in-house, teamed up with HHCL/Red Cell in 2003 to create a mini feature film with an all-star cast in an attempt to galvanise support for its cause.