The stark warning comes amid a torrid time for the high street as consumers stop spending amid the Brexit chaos.
The Wakefield-based firm said the uncertainty surrounding Brexit is a significant factor affecting demand and it may not pick up until the current period of heightened uncertainty ends.
The firm said that as it has no visibility of when matters will be resolved, it has taken a cautious approach to forecasts and assumed that sales will not show any significant improvement before the end of March 2019.
As a result, and partly to reflect the high degree of uncertainty inherent in the forecast, it now estimates that underlying pre-tax profit will be in the range of break-even to a loss of £4.0m for the current financial year.
It said the mid-point of this range corresponds to a store like-for-like sales assumption for the third quarter of minus 12 per cent and minus 1 per cent for the fourth quarter, against a particularly weak comparative last year of minus 11 per cent.
Bonmarche's chief executive Helen Connolly said: "The current trading conditions are unprecedented in our experience and are significantly worse even than during the recession of 2008/9. I hope that in the fullness of time, our cut to the forecast may prove to have been overdone, but in the current market, this seems the appropriate stance to adopt.
“I believe that Bonmarche is well prepared to weather the storm, and that we can look forward to some recovery in 2020. Accordingly, the board remains confident in the strategy, and in the company's long-term prospects."