By Nicola Carslaw
BBC consumer affairs correspondent
Marks and Spencer, which has the UK's third largest number of High Street coffee shops, has switched all the coffee it serves to Fairtrade.
Fairtrade offers coffee producers in the developing world a better deal
It is the first UK chain to do so, and it is estimated that the move will double the amount of Fairtrade coffee on sale in the nation's coffee shops.
Fairtrade means farmers in developing countries get paid more than the market price for their beans so they can improve their working and living conditions.
People in the UK are the world's largest consumers of Fairtrade foods.
But, if you ask shoppers what they know about it, there's a high chance many will tell you they have never heard of it.
Now, however, that could change.
Jimmy Navarro is a coffee grower in Honduras and the cooperative he leads is one of M&S's main suppliers.
This week he has come to Britain to meet the shoppers actually drinking his product.
Some of them were bemused at first. But they all wanted to know more.
What is Fairtrade and what difference does it make to farmers like him?
"We are paid a better price and we have drinking water, because we get paid direct and can choose how to spend the money within our communities," Mr Navarro told them.
"The only way to change the world and for everyone to have a half-decent life is for people here to pay a decent price for our coffee."
Other chains already offer limited Fairtrade options or coffees marketed as ethical. But campaigners say that to switch all coffee to Fairtrade without putting up the price is groundbreaking.
Harriet Lamb heads up the charity the Fairtrade Foundation. It licenses goods so they can qualify for a Fairtrade label.
"What Fairtrade does is it gives farmers the choice to stay on the land, to send their children to school and to invest in improving the quality of their coffee," she says.
"Uniquely it gives them the chance to participate in international trade and, indeed, to become businessmen."
Sales figures suggest that switching to Fairtrade can make sound business sense.
The Co-op's own brands of coffee and chocolate are all Fairtrade. Since buying direct from Fairtrade cooperatives they have seen sales in their shops soar.
Chocolate sales volumes were up 24% in the 12 months following conversion to Fairtrade and a further 46% in the first half of this year. In both periods, sales of other more established brands have stayed static.
But there is a note of caution sounded by Paul Gostick, the international chairman of the Chartered Institute of Marketing.
"Consumers say in research questionnaires that they are prepared to pay a premium for ethically produced goods. The reality is that not as many will actively part with their money when asked to do so," he says.
Marks and Spencer says that as Fairtrade coffee is now served as standard and at no extra cost, its customers will not need to part with more money - they will just enjoy a tasty cup of coffee.