Dairy farmers from Lincolnshire are joining hundreds of others from across the UK to fight for a fair price for their milk.
They are angry at the plummeting price of milk and are heading down to the capital today for talks on how to fight back.
From August the big 3 milk processors, which supply it to the supermarkets, want to pay farmers an all-time low cost of 24.5p a litre.
But dairy farmers say it costs them around 30p to produce a litre of milk, so they'll be forced to operate at a loss.
The NFU estimates it couild leave farmers out of pocket by up to £40,000 a year.