Battle lines have been drawn in an effort to protect one of the region's most valuable crops – but the controversial relaxation of an EU pesticide ban could put Norfolk's farmers at a disadvantage against their county neighbours.

Last week, an emergency temporary approval was granted for the most at-risk oilseed rape growers to use neonicotinoid seed treatments – protective chemicals which were banned two years ago due to fears over their impact on the health of bees.

The decision was branded as 'scandalous' by bee campaigners at the Friends of the Earth, who have now launched a legal challenge against the 120-day derogation.

Meanwhile, the National Farmers' Union (NFU) has issued advice on how the limited approved quantity of treated seeds will be distributed.

The treated seed will targeted at 30,000 hectares in Suffolk, Cambridgeshire, Bedfordshire and Hertfordshire – the four counties worst affected last season by cabbage stem flea beetles, the pest traditionally controlled by neonicotinoids.

But those boundary lines have caused frustration for Norfolk farmers.

Chris Eglington, who grows 200 acres of oilseed rape at Letton, near Shipdham, discovered an infestation of flea beetle larvae in the roots of his plants earlier this year, but he says he won't know the full extent of the damage until he has harvested his crops.

'In a word, I'm disappointed,' he said. 'It is a great pity that we are not in that area. I don't know how bad we are compared to those other areas where the crop has been completely lost – I won't know that until we cut our rape, which is probably still a week away at the moment. 'I think those people lucky enough to get neonicotinoids will be able to protect their crop and those that have not will be in a very similar position to the areas that were really poor last year. The beetles are building up, and I cannot see it getting any better.'

Stephen Rash, the NFU's Suffolk council delegate, farms just south of the county border at Wortham near Diss, so he is 'very hopeful' of getting treated seeds to plant this autumn.

Although his winter oilseed rape is in good condition, he says his entire spring-sown crop – 11 hectares of it – was completely destroyed by flea beetles.

'It is going to put a serious dent in our profits,' he said. 'It is a small proportion of our total cropping, but no loss is a good loss.

'The green lobby has mounted a very emotive campaign but the evidence for the ban is pretty poor. Worldwide, these chemicals are still being used and we are at a disadvantage to our competitors.

'I'm very hopeful that my seed merchant will be able to get some of this dressed seed, but quite what the difference is between me and my neighbours across the river, 500 yards away in Norfolk, I don't know. I do feel sorry for my compatriots. For them to not have access to it is going to make things very difficult.'

Cabbage stem flea beetles are widespread in the UK and northern Europe. They migrate into oilseed rape crops soon after emergence, chewing through leaves and causing 'shot-holing' symptoms, which can result in stunted growth and, in severe cases, can even kill the seedlings before they emerge.

The beetles have traditionally been controlled using neonicotinoid seed treatments.

Environmental campaigners point to scientific studies showing that these insecticides have serious, harmful effects on bees and other pollinating insects, while farmers argue there has not been enough data from realistic field trials for a fully science-based decision to be made.

This year's harvest of winter oilseed rape is the first not to have been treated with neonicotinoids. The ban was agreed in 2012 and introduced in December 2013, affecting winter oilseed rape crops planted in July/August last year.