From Fairtrade Fortnight (February 26 to March 11) the retailer will stock nearly 70 different items, including womenswear, menswear, childrenswear and baby clothing.
The products will be available in 100 stores across the UK, Republic of Ireland and also online. Women's strappy vests will be£12 for two, men's T-shirts will cost£7 and formal shirts will be£19.50. Girls' vests will fetch£4 to£5, boys' hooded sweatshirts will be£10 to£12 and baby two-piece outfits, which are also organic, will cost£10.
New homewares ranges include Fairtrade cotton sheets, pillowcases, towels and tea towels, ranging from£2 to£24. All items will carry a Fairtrade label to show the cotton has been certified by the Fairtrade Foundation.
Marks & Spencer chief executive Stuart Rose said: 'The demand from our customers for fair trade products is getting stronger. Our initial Fairtrade cotton range proved so popular last year that we've extended it and are now the biggest retailer of Fairtrade cotton on the high street.
'As part of our five-year eco-plan, Plan A, we will bring even more Fairtrade cotton clothing into our stores, converting entire ranges like our basic men's and women's T-shirts. This will add up to about 20 million garments and will account for about a third of the world's current supply of Fairtrade cotton.'
Last year, the retailer converted all its tea and coffee to Fairtrade, which meant that more than£340,000 in Fairtrade premium went directly back to the farmers. Tea and coffee sales at M&S increased 5 per cent over the year against a declining market.
Fairtrade Foundation executive director Harriet Lamb said: 'The growth of the Fairtrade product range in M&S over the past year has been truly remarkable.'