At the Hothouse event in Bristol last week, Porritt said cities have a mandate to promote change. But he added that the public must be convinced that such change will benefit them now.
Cities have to make a case for "what the benefits are today rather than doing things now for the sake of the future," he said.
Low-carbon building stock to eliminate fuel poverty is one of the fastest routes to securing social justice in urban areas, he argued.
Porritt also warned that the economic competitiveness agenda is incompatible with sustainability. "If in the next ten to 15 years any city starts building new urban motorways, it will be out of its tiny mind," he claimed.