Companies must ensure workers receive fair price for their labour
London has become a Fairtrade city after most businesses were found to offer products bought on fair trading terms from the developing world.
About 1,000 retailers and 600 catering outlets, which includes the Houses of Parliament, sell items carrying the Fairtrade mark.
London is the biggest city in the world to get the citation, the Fairtrade London Steering Committee said.
London Mayor Boris Johnson said the 20 boroughs deal in fairly traded goods.
In order to be granted Fairtrade status, companies must ensure that all those involved in the production process, at every stage, receive a fair price for their labours.
London Fire Brigade, the Metropolitan Police, Bank of England and numerous museums, universities, churches and school sell Fairtrade goods to their staff and visitors.
London is the 400th town or city in the UK to be awarded Fairtrade status.
Sophi Tranchell, chairman of the Fairtrade London Steering Committee, said: "Now London has achieved Fairtrade City status we can really stake our claim as the Fairtrade capital of the world."
Mr Johnson said: "It's super news that London is now officially the world's largest Fairtrade city, with 20 Fairtrade boroughs in the capital.
"It sends a clear message that Londoners want their products to be Fairtrade and to make a significant difference to the lives of farmers and workers in the developing world."