Christian environmentalists are calling on the public to buy Fairtrade products after sales fell for the first time in 20 years. The drop of nearly 4 per cent in sales of Fairtrade products reflects the success of cheap brands offered by Aldi and Lidl at a time when the government's austerity cuts are affecting hundreds of thousands of people.

Dr Ruth Valerio, churches and theology director of A Rocha UK, the Christian research, environmental and conservation charity, said switching to Fairtrade could change people's lives.

The foundation was set up to help provide a fair deal for farmers and producers in the developing world.

Dr Valerio told Christian Today the fall in sales should not be ignored and she urged everyone to act.

"With the changing shape of supermarket shopping and the squeeze that is on the grocery sector, it highlights even more how important it is for everyone to choose products that change people's lives," she said.

"So I would urge people to look for the FT label on products and to make a conscious effort to buy them whenever that option is available - and to ask your supermarket to stock more FT lines."

She added: "The good news is that sales of FT gold is bucking that overall trend and, since the launch of the I Do campaign, FT gold licencees have reported huge increases in sales. That is absolutely fantastic news because I know that by choosing FT gold we are changing the lives of people, their families and the environment that they're a part of."

The Guardian reported that the retail sales value of products overseen by Fairtrade, which aims to protect farmers in developing countries by promising them a minimum price and a premium to invest in community projects, fell by 3.7 per cent last year to £1.67bn.

Michael Gidney, chief executive of the Fairtrade foundation, said: "Our main concern now is that increasingly aggressive competitive behaviour in the grocery sector could undermine the volumes farmers and workers are able to sell on Fairtrade terms. This will result in real losses to hardworking families and communities in some of the poorest countries in the world."

Fairtrade products are also thought to have been hit by falling sales at Sainsbury's and the Co-op, which are both important outlets for the products and stock Fairtrade bananas, coffee, tea and chocolate.

Larry Bush, marketing director of Traidcraft, which sells the UK's widest range of fair trade products, said: "It's obviously disappointing to see Fairtrade sales fall for the first time in 20 years. This is linked to consumers shopping more in discount retailers where there is less choice of Fairtrade products than in retailers like the Co-op and Sainsbury's. However, the UK is still the world's largest market for Fairtrade and there is still huge support for Fairtrade. There's still a real need to support Fairtrade and by buying from Traidcraft you can do this in a way that helps particularly marginalised producers."