The Prince of Wales has been asked to take a leading role in protecting his estate's workers from "attack", if the government's plan to abolish a board that sets working conditions for agricultural workers goes ahead.
The Trades Union Congress (TUC) and Unite have sent a joint letter to Charles in which they call on him to defend his estate's agricultural employees and protect their pay.
The TUC and Unite have previously written to Prince Charles over the AWB
The Agricultural Wages Board (AWB) for England and Wales sets legal minimum pay and conditions - including overtime rates, holiday pay and a sick pay scheme - for 150,000 farm workers and is a benchmark for thousands more rural workers.
The government plans to abolish the AWB despite support from farmers and landowners.
More than 60% of those replying to a government consultation called for the AWB's retention, Unite said.
In the joint letter, the unions said: "We would propose that the estate might take a leading role in defending the terms and conditions of its agricultural employees and market gardening employees from attack.
"This might include giving appropriate advice to tenant farmers and to managers in the Duchy's businesses."
Clarence House said it does not comment on private correspondence.
TUC general secretary Frances O'Grady and Unite general secretary Len McCluskey previously wrote to Charles raising their concerns over the comments by the general manager of the Duchy of Cornwall Nursery, part of the Prince's estate.
Responding to the government consultation, Angie Coombs called for the AWB's abolition, calling it "archaic and not fit for purpose".
A Duchy of Cornwall spokesman said: "The response submitted by the Duchy Nursery represented its view as a commercial business in the field of horticulture, not the Duchy as a whole or the Prince of Wales."
A Government amendment on the abolition of the AWB comes to the Commons on Tuesday 16 April.