Stella McCartney will help Vivienne Westwood create sustainable fashion.
The designer has vowed to support her rival by working alongside the luxury fashion brand to create fashion that is good for the environment.
She said: "I'm going to help you and I can't wait to work with you.
"I would gladly give you my information sheet and will be very proud to do so."
The 43-year-old style guru - who is a lifelong vegetarian and refuses to use leather or fur in her designs - also spoke to designers about taking "small steps" to ensure they could help make fashion a sustainable business.
Addressing the crowd as part of a talk organised by Business of Fashion, she said: "I'm a big believer in something is better than nothing.
"The smallest thing can have an impact. We are all living on this land with limited resources, and we all have to be conscious about how we consume and how we manufacture and how we source - and the fashion industry is no different than anyone else.
"Every industry has to be mindful of how they go forward in their business and sadly luxury fashion is incredible harmful - and fashion in general. And it needs to just a get a bit more modern and man up a bit."