The NFU's emergency application for use of neonicotinoids has been refused by Defra.
In May, the NFU said it would continue to apply for the emergency use of neonicotinoid seed treatments on behalf of farmers facing pressure from cabbage stem flea beetle.
The NFU submitted two revised applications on June 1 to the Health and Safety Executive for emergency authorisation to allow the use on oilseed rape of Cruiser OSR and Modesto, which contain neonicotinoids.
Reacting to today's news, NFU Vice President Guy Smith said: "This is very disappointing news for oilseed rape growers."
"This will be a bitter blow for farmers at a time when oilseed rape is one of the few arable crops with a good, positive margin Guy Smith, NFU
"If we have a difficult establishment period this autumn with high flea beetle pressure, I have no doubt that a huge percentage of the oilseed rape crop will be lost.
"This will be a bitter blow for farmers at a time when oilseed rape is one of the few arable crops with a good, positive margin."
'Great news for bees'
The move has been 'warmly welcomed' by Friends of the Earth.
Friends of the Earth bee campaigner Dave Timms said: "This is great news for bees and other wildlife. We are delighted the application has been refused.
"Despite having the advantage of secrecy, the evidence submitted by the NFU failed to convince the Government’s pesticide advisors.
"The NFU risks damaging public trust in British farming with its repeated attempts to get these dangerous pesticides back into our fields – instead they should concentrate on promoting bee-friendly ways to control crop pests.
"The Government must do all it can to safeguard our under-threat pollinators.
"This should include maintaining the current ban on bee-harming pesticides - and committing to upholding and enforcing EU nature protection rules, which are now at risk as we plan our Brexit."