When choosing between the big coffee chains, consumers should also consider the wages of the workers and the ethics of the company

Walk down your local high street you're likely to be presented with a choice between the big three UK coffee shop chains – Starbucks, Costa or Caffè Nero. Only Starbucks sells Fairtrade coffee. So would this be my about-town cup of ethical choice? Not after I'd researched the ethics of coffee shop chains for Ethical Consumer magazine, rating them across 19 ethical categories from workers' rights to antisocial finance.

I would always reach for a bag of Fairtrade coffee from the shop shelf over anything else. But if the choice on the high street was between a Fairtrade espresso from union-busting, Guantanamo Bay-supplying, trademark colonialists Starbucks - or a Rainforest Alliance espresso from Costa - I know which I'd choose.

And that's not because I'm sold on Rainforest Alliance certification. Both schemes involve minimum social, labour and environmental standards. The key difference is that Fairtrade guarantees a minimum price that tracks slightly above market rates, plus a "Fairtrade premium" that can be invested in projects that enhance social, economic and environmental development. The Rainforest Alliance label only guarantees that 30% of coffee beans in a product have been certified.

Fairtrade is still the gold standard. And our Ethical Consumer Best Buy for coffee shop chains goes to AMT Coffee - the first UK coffee shop to go 100% Fairtrade with its coffee, and offer 100% organic milk.

With a multi-million pound advertising campaign in the UK stressing its ethical credentials, you could easily be forgiven for thinking that Starbucks had gone entirely Fairtrade. Since 2009 its coffee in the UK and Ireland is Fairtrade, making it the world's biggest buyer of Fairtrade-certified coffee. But in the US it's another story. It took a boycott campaign of many years by the Organic Consumers' Association (OCA) to shame Starbucks into promising to brew a Fairtrade coffee in any of its stores. And the OCA continues to criticise the world's biggest coffee shop for dragging its feet on Fairtrade on that side of the Atlantic. But selling Fairtrade coffee in itself does not an ethical company make – Starbucks comes bottom of Ethical Consumer's rating table.

In addition to the reasons I've already listed above, there's the ugly business of a US court ordering Starbucks to pay more than $100m into the accounts of its low-wage staff in California after ruling that it had improperly required the workers to share tips with their bosses, although the ruling was subsequently overturned on appeal. Which I'm sure didn't put chief executive Howard Shultz out of pocket. In 2009 he received a 25% pay rise – a year in which the company slashed costs by $580m. As for that green-tinged image? It took campaigners six years before Starbucks stopped serving up the genetically engineered artificial "recombinant bovine growth hormone" in its milk in the US.

So what about Costa – second from the top in Ethical Consumer's coffee shop ratings table. It's owned by Whitbread, the UK's largest hotel and restaurant group. Whitbread's brands - like Premier Inn, Beefeater and Table Table – aren't those you'd immediately associate with ethical consumerism. So it's a pleasant surprise that in 2009 the company launched an environmental and labour rights programme with best practice policies.

And Caffè Nero? It's a sign of how Fairtrade has entered consumer consciousness that Caffe Nero feels the need to put "Fair Trade" prominently on its website although they don't actually sell Fairtrade coffee (spot the difference?). In fact the company was unable to provide any information regarding its "Fair Trade" claims.

Ethical consumers will of course want to support coffee producers with Fairtrade. But they might spare a thought for the poverty wages of the coffee shop worker and the ethics of coffee shop company too.

• Dan Welch is the co-editor of Ethical Consumer magazine. The Coffee Shops Buyers' Guide is available in the latest issue of Ethical Consumer magazine

• 2 March update: "although the ruling was subsequently overturned on appeal" was added