Mr Roberts, whose appointment was announced in March, was due to take over next month from Robert Forster who is due to retire on August 7.
He advised NBA chairman, Duff Burrell, on Monday that he had changed his mind and was joining the BMPA as director in succession to Maurice McCartney who left on May 31.
“It is a shock and I’m disappointed but not beat,” said Mr Burrell. “Processors keep telling us they want to build up a good relationship with suppliers but this blatant poaching is not helping the fragile relationships in the industry.”
It is understood Mr Roberts has been offered a much more attractive financial package to join the BMPA.
“The abattoir owners have always had more money than farmers,” added Mr Forster.
Mr Roberts is due to leave his job at DEFRA, where he is currently working on cost and responsibility sharing, on July 6. He previously worked closely with the meat trade on the development of the stem testing regime which has replaced the Over Thirty Month Slaughter scheme.
The NBA post is likely to be re-advertised although Mr Burrell said there were a number of options open to him.