The latest trading update for the Birmingham-based business shows sales for the 15 weeks to January 8 were down 1.5 per cent.
Closures of sites due to Covid have contributed to the decline.
Mitchells & Butlers, which includes Harvester, Toby Carvery, All Bar One, Miller & Carter and Stonehouse, said it made a strong start to the year with like-for-like sales growth of 2.7 per cent over the first eight weeks.
This encouraging performance continued until early December when concerns first arose around the emergence of the new variant, leading to calls for further caution in socialising which resulted in a downturn in activity across the sector.
Over the last seven weeks sales have been down six per cent, with the adverse impact of Omicron being particularly felt in the most recent four weeks, over the important festive season, during which like-for-like sales have been down 10.2 per cent.
Chief executive Phil Urban said: "This first quarter performance represents a robust performance given the challenges the industry faces from the rapid spread of the new variant both in terms of reduced consumer activity and disruption caused by the inevitable isolation of team members.
"Experience shows that as restrictions ease, and confidence returns, our business is able to swiftly recover. To that end, whilst we expect activity to continue to be adversely impacted in the short term, we are encouraged by the latest data on the Omicron variant which we believe will boost consumers' confidence to return to pubs and restaurants allowing us to regain the momentum which was beginning to build."