The London rally due to take place on 19 November is now as 'full capacity', and unregistered farmers are being told not to travel to the capital.
Organised by the NFU, the 'mass lobby of MPs' will take place at the Church House conference centre in Westminster, on Tuesday 19 November.
It is taking place due to inheritance tax changes to agricultural property relief (APR) and business property relief (BPR), with fears this would deal a 'hammer blow' to family farms.
In a new update, the NFU said that demand had been 'amazing', with a total of 1,800 farmers expected to arrive in a bid to lobby MPs to make changes.
The original capacity was 600, which is what the venue holds, but the NFU had to triple this to 1,800 via three rotations due to high demand.
Because of this, farmers not registered have today (7 November) been told not to travel to London, as the event was now at 'full capacity'.
"We can’t make it any larger," the NFU said in a letter to members, adding that there were also 'legal issues' to consider.
"We can’t simply turn up in numbers in Westminster on the streets, or the open spaces," the union explained.
"We cannot risk either member or public safety, or the loss of public support, that could come from what could be an illegal demonstration."
However, the NFU said that the event would not be the only one, as others would be planned for the future if changes to APR and BPR are not made.
"We are asking members who have not registered not to do so now, we want this to be the first event, not the only event, where you can be heard," the letter said.
"Far more than 1,800 NFU members want their voice to be heard, and rightly. The level of anger in the industry may never have been so high."
Registrations for the London rally was launched only last week, after the union warned the government that its autumn budget changes to APR and BPR needed to be 'overturned, and fast.'
Tightening margins, record inflation, extreme weather and increased production costs meant that many farmers were now at 'breaking point' and 'unable to absorb any more cost burden'.
The tax changes could also increase food costs to consumers, it warned, adding pressure to many still experiencing the cost-of-living crisis.
The NFU's new letter to its members concluded that, despite the rally being at full capacity, there would be "another opportunity to make clear how you feel."
"We’ve always said this event was just the start of this fight - if the government listens to us, before or after the 19th, we’ll all be relieved," the NFU said.
"But if it doesn’t, this event will be followed by another, at which farmers and growers will demonstrate how we feel about this devastating policy.
"If we need it, that will be our show of mass unity and strength."
