Advocates of genetic modification are like Jehovah's Witnesses. You tell them, politely but firmly, that you're not interested and shut the door on them. So do all your neighbours, apart from the mad professor down the street who has no-one else to talk to. A few years later, they're back on your doorstep telling you they still have the only answer to the latest world crisis. 

Like peripatetic evangelists, GM lobbyists never give up. They got well and truly cuffed in Britain when they had their last surge of activity in 1999. Now they are back, using anxieties over food security to get a toe in the door and brandishing a purple tomato, the latest GM miracle that's going to prevent cancer - without any downside whatsoever, naturally.

While Jehovah's Witnesses tell us faith in their gospel will save the world, GM zealots are still touting a fairy wand solution to ill-health, malnutrition and global warming.

It seems hard-hearted to send away fanatics without pretending to listen to their best argument. For the GM boys, their strongest hand used to be that GM was grown and consumed extensively in the US, without any fuss or fall-out.

This was a half-truth. GM food in the US had been sold unlabelled, so consumers were in the dark about what they were buying.

Now America is turning against GM. Sales of milk have plummeted due to animal and human health fears about Monsanto's GM hormone rBGH, sold to increase milk yield. The Canadian Medical Veterinary Association reported that it caused a 50% increase in painful lameness in cows and a 25% increase in mastitis. No wonder its use fell by 23% from 2002 to 2007. Resistance from US farmers and regulators has stopped GM rice and wheat getting beyond the field trial stage. Approval for GM alfalfa has been withdrawn.  

American ennui with GM echoes attitudes worldwide. Fifty-four out of 60 countries have agreed with the International Assessment of Agricultural Science and Technology for Development (IAASTD) report that did not foresee a major role for GM.  The Republic of Ireland, Northern Ireland, Scotland and Wales all have GM-free policies.

Only the English Government is impressionable enough to give the GM ideologues a hearing. It's time Defra shut the door and packed them off down the street, munching their purple tomatoes.


Joanna Blythman is a food journalist and author of Bad Food Britain.