Bosses at the Wolverhampton-based group said like-for-like sales had been rising and were up 1.3 per cent in the eight weeks to November 27, but they fell sharply in the following eight weeks to January 22 as the virus spread – falling 8.8 per cent compared with the same period two years ago.
In total, like-for-like sales for the 16-week period were down 3.9 per cent, the group said ahead of its annual general meeting.
Bosses said the falls reflected "the impact of the Omicron variant and consumer sentiment related to the new variant... as a result of Government messaging including guidance to work from home and the call to limit social distancing", with pubs in London suffering worse than those elsewhere in England.
The falls were more pronounced in Wales and Scotland, which faced stricter restrictions than England, although the company said it remains confident for the future.
During the five weeks of December, like-for-like sales compared with the market outside the M25 were one per cent ahead and total sales were five per cent ahead.
Chief executive Andrew Andrea said: "Whilst the emergence of the Omicron variant and subsequent Government guidance temporarily impacted consumer sentiment, we remain confident that the strong trading momentum which we were experiencing prior to that will resume.
"We welcome the various plans under way to gradually ease trading restrictions in Scotland and Wales.
"These, together with the reduction in the required self-isolation period and anticipation of an imminent end to the work-from-home directive, should enable some semblance of normalised trading patterns to return.
"Indeed, there is growing evidence over the most recent of weeks of the new year that consumer confidence is rebuilding, and guests are returning to our pubs in greater numbers, which is encouraging.
"Importantly, Marston’s has a well invested, predominantly community pub estate which is well placed to benefit from the pent-up consumer demand which we are confident remains.”
At the start of the year Marstons' bank borrowings were £199 million against the Group’s £280 million bank facility, which is in place until March 2024.