The GMI - which includes retailers such as B&Q, producers such as Scotts and anti-peat groups including the RSPB - has allowed Bord na Mona to be a member despite the Irish peat board using 97 per cent of its peat as fuel for power stations.
Briercliffe said: "The GMI covers the UK market and can't dictate what happens in other countries. It would be a different matter if Bord na Mona supplied power stations in this country, but it doesn't."
RSPB sustainable development policy officer Olly Watts said: "We discussed long and hard whether or not Bord na Mona should be a GMI member. We decided the power station part of the equation should be attacked by the green energy lobby. The horticulture industry should not have to shoulder that burden."
Electricity sourced from burning peat is "the dirtiest way of making energy", Watts added.
Meanwhile, B&Q is still selling 100 per cent peat growbags sourced from Bord na Mona despite TV gardener Chris Beardshaw backing the retailer's PR campaign, which says it has stopped selling 100 per cent peat bales.
Watts said: "If they're doing 100 per cent peat growbags, that's disappointing. Growbags are a good opportunity from a technical point of view to use alternative materials. It is a pity they are doing anything 100 per cent peat at all."
Bord na Mona did not comment.