A spokesman for the EU's agriculture commissioner has said it is unlikely a £60m fine imposed on the Department of Agriculture will be reduced.
The NI Assembly's agriculture committee is in Brussels to press for the reduction of the fine.
It was imposed due to farm subsidy payments made in error to farmers in Northern Ireland.
Roger Waite said he could not see the commission changing its mind on the size of the fine.
"There is an appeal and a conciliation process going on which will only be finalised by the end of the year and so legally speaking I can't comment on that," he said.
"But what I can say is that in the past the commission has been absolutely firm and cannot change the amount of a fine that is imposed."
The agriculture minister, Michelle Gildernew, has said her department is willing to go to the European courts to fight against the fine.
The errors were discovered when European auditors carried out spot checks on individual subsidy claims made by farmers.
The committee members will argue that it is a disproportionately heavy fine.
The audit found that, in some instances, more than one farmer was claiming subsidies on the same piece of land.
However, Alliance MLA Stephen Farry said someone must take responsibility for the original error.
"Accountability is important in a democracy," he said.
"Clearly there is an issue of how forms have been filled out by farmers, but there are also issues about the administration of the department as well.
"This is a major statement of poor financial management from the European Union, directed towards a government department in Northern Ireland.
"This may have major consequences for the rest of the Northern Ireland budget at a time of major financial pressure coming from the UK treasury."