The UFU comments follow reports that the gap between prices paid to local beef farmers and producers in GB widened during the month of April.
UFU Beef and Lamb Policy chairman William Taylor said farmers had hoped the 'price differential' issue was a thing of the past.
Taylor said: "Our beef is being produced to the same UK standards and is being sold in the same UK market, so there has never been any justification for prices in Northern Ireland being lower than in other parts of the UK.
"This is an issue which the UFU has repeatedly raised with meat plants and retailers and progress was being made as the gap had narrowed.
"It is very worrying to see the gap widen again during the month of April and it is something that the UFU will continue to highlight as an unacceptable practice in the industry. If we are to have a long-term sustainable supply of local beef, then producers in Northern Ireland will have to be treated fairly."
The UFU highlighted that while average farmgate prices for Northern Ireland beef rose by 7p/kg during April, prices in most GB regions rose by about 14-17p/kg in the same period.