Britain's most popular chocolate bar, Dairy Milk, is to become Fairtrade certified in Britain and Ireland in a move that will double the amount of cocoa bought from smallholders in the developing world under the sustainable farming scheme.

The decision - the highlight of Fairtrade Fortnight - will transform Fairtrade chocolate from a niche preference to a mainstream ethical benchmark, putting the sustainability quality mark on 15% of chocolate sold in Britain. Cadbury's chief executive, Todd Stitzer, said he plans to convert the group's other chocolate brands to Fairtrade "as soon as we can do it". Dairy Milk is the first mainstream chocolate bar to be sold with a commitment to pay cocoa suppliers the "Fairtrade premium" of $150 (£105) a tonne above market prices. When the bars go on sale this summer the value of Fairtrade chocolate sales in Britain will leap from £45m to £225m. Cadbury's pledge to buy 10,000 tonnes of cocoa under Fairtrade terms will triple certified sales from Ghana.

Fairtrade terms require buyers to commit to a minimum price of $1,600 a tonne. For more than two years the open market price has been climbing and yesterday reached $2,213. This is the longest period prices have stayed above the guarantee price and the International Cocoa Organisation yesterday predicted a third year of "production deficit". This makes a Fairtrade commitment more affordable than in previous years. Stitzer nevertheless insisted Cadbury was committed to Fairtrade for the long term, regardless of price movements.

• This article was amended on Wednesday 30 September 2009. Fairtrade is a quality mark; it is not, as we said, a kitemark. Kitemark is a registered trademark for products and services audited by the British Standards Institution. This has been corrected.