Farmland equivalent to the entire arable area of Norfolk will have to be mothballed if Common Agricultural Policy reform proposals remain unchanged, according to farmers' leader Peter Kendall.

This is the stark warning that the president of the National Farmers' Union will deliver to Norfolk farmers when he addresses the NFU's annual county meeting on Thursday.

Mr Kendall will set out the impact of the European Commission's proposed changes – due to take effect from 2014 – and urge members to fight for a fairer deal by lobbying MPs and MEPs.

He will tell farmers at Easton College how NFU economists have been faring on one aspect of the proposals, the suggestion that seven per cent of farmland should be managed as an 'ecological focus area'.

Seven per cent is about 420,000 hectares of land in the UK, and the economists estimate that 120,000 hectares of that is likely to be found from uncropped land.

'That leaves 300,000 hectares of land to be mothballed – an area the size of Northamptonshire, the equivalent of all the arable land in Norfolk,' Mr Kendall will say.

'According to analysis by the Agriculture and Horticulture Development Board, the effect will be to cut gross margins by �200m a year.

'That's not just �200m out of farmers' pockets, it's �200m out of the economy, at the time when we can least afford it.'

The NFU is particularly concerned that the so-called 'greening' requirements within the CAP reform proposals will undermine voluntary efforts to improve the environment.

It believes there is a real danger that English farmers will be placed at a competitive disadvantage if they are asked to do more on top of existing environmental commitments.

In England, more than two thirds of arable land (68pc) is currently covered by agri-environment schemes.

Mr Kendall will tell the meeting that the objective of greening the CAP must be to bring all farmers in the European Union up to the same level as the better performing countries. It should not compel those farmers who have already made significant efforts for the environment to do more.

During the meeting, NFU county chairman and Breckland farmer Francis Ulrych, of Griston, near Watton, will update members on NFU county activities during 2011 and farmers will elect county representatives for regional commodity boards covering livestock, combinable crops and dairy.

The meeting starts at 7.30pm and it is open to all NFU members in Norfolk.