The prime minister has pledged to stress the importance of the need for an assessment before the proposals become law next year.
In a letter to the All-Party Parliamentary Group on Science & Technology in Agriculture, Brown wrote: "We remain concerned... and we will continue to stress the need for a proper understanding of the impacts for the whole (European) Community as the negotiations progress."
His calls for an impact assessment come as the Pesticides Safety Directorate (PSD) has revealed a revised report on products that could be lost.
The latest PSD report has been published in light of the EU pesticides legislation's second reading amendments agreed by the European Parliament's environment committee.
The PSD looked at 278 substances and concluded that 14 to 23 per cent could be lost as a result of the proposals. The report said: "It is clear it is not simply the percentage or absolute number of substances that might be lost which is the most important factor, but the particular substances concerned."
The PSD is now working on a comprehensive report that will outline the impacts for conventional crop production in the UK. Policy officer Mark Hawkins told HW the report is expected to be published today.
NFU plant health adviser Paul Chambers said despite the reduction from the 85 per cent of substances lost revealed in the previous PSD impact assessment, the outlook was still bleak: "In terms of herbicides, horticulture will still be hard hit."
"It depends what substances are lost, not the figures," he added.
There is still uncertainty over what the final legislation could contain as European Council, Commission and Parliament representatives have not yet managed to resolve their differences during ongoing trialogue meetings.
The aim of the meetings is to reach a compromise between the council and commission position and the more stringent proposals from the parliament's Environment Committee, led by the German Green MEP Hiltrud Breyer.
Amenity Forum chairman Jon Allbutt said: "The situation is serious but I remain optimistic the lunatics won't take over the asylum."
East Malling Research horticultural research leader Jerry Hall added: "It all depends which criteria are actually chosen - some insecticides are very important to certain sectors of the industry."
Rothamsted Research has delivered a petition to the European Parliament and 10 Downing Street.
Further trialogues are expected this week and, if agreement is not reached, a final meeting could take place on 23 December.
Chambers added that it was almost certain that the Environment Committee's proposals would have to be diluted for agreement to take place.
A full plenary vote is still expected on 12 January.
Crop Protection Association chief executive Dominic Dyer said: "We remain deeply concerned about the impact of these proposals.
"We continue to call on the UK government to seek agreement with other member states to include a safeguard clause in the final regulation, setting up a council work group to carry out a full EU-wide assessment on this legislation before it comes into law."
HTA business development director Tim Briercliffe agreed: "We believe that horticulture will be particularly badly affected by the proposals as, although only about 20 per cent of products are likely to be affected, the sector will struggle to control up to 80 per cent of the diseases, pests and weeds that affect gardens."
He added that the HTA will continue to campaign for an impact assessment on the legislation before the January vote.