Nestle sells more Kit Kats in the UK than anywhere else in the world
Kit Kat, Britain's biggest-selling chocolate biscuit, will bear the Fairtrade logo from mid-January.
The biscuits, made by Nestle in York, have been Fairtrade certified, with more than 6,000 farmers in west Africa, set to get a better price for cocoa.
About one billion Kits Kats are sold in the UK every year and Nestle said it was committed to helping cocoa farmers.
The Fairtrade mark already appears on more than 4,500 products in the UK but campaigners hope more will follow.
'Tip the balance'
Harriet Lamb, of the Fairtrade Foundation, estimated the Ivory Coast farmers would receive hundreds of thousands of pounds more next year.
"The significant volumes of cocoa that go into making Kit Kat will open whole new possibilities for these farmers, giving them a more sustainable livelihood and the chance to plan for a better future," she said.
Cadbury's Dairy Milk, the UK's best-selling chocolate bar, was Fairtrade certified in March this year, and Ms Lamb hopes the move by Nestle will start to "tip the balance" in cocoa trade.
Eileen Maybin, also of the Fairtrade Foundation, said it hoped other companies would follow the example of both Cadbury's and Nestle.
She said: "This will challenge other companies to follow suit.
"Nestle are one of the biggest food and drink companies in the world. Companies watch what each other is doing and we would expect other companies to be monitoring what a company like Nestle does.
"If Nestle thinks Fairtrade is a viable option then hopefully other companies will see it as something they should be getting involved with themselves."
David Rennie, managing director of Nestle Confectionery, said the company sold more Kit Kats in the UK than anywhere else.
"UK consumers are increasingly interested in how we source and manufacture their favourite products and certifying our largest and most iconic brand is one of the ways in which we are committing to improving the lives of as many cocoa farming families as possible," he said.
Trade and Development Minister Gareth Thomas said: "This will give thousands of Ivorian cocoa farmers better opportunities to trade their way out of poverty."
Launched in the UK 15 years ago, the Fairtrade campaign offers farmers in developing countries a better price for their produce.
Under the terms, farmers receive a guaranteed minimum price plus a premium of more than £100 a tonne, which is used for business or social development products.
Last year more than £700m was spent on Fairtrade goods in the UK, an increase of more than £200m on 2007.