For the first half of 2011, the Co-op saw a food sales decline of 4.6% compared with last year. Like-for-like sales were also down 3.6% compared with 2010. The group's operating profit for the six months was £135.4m, down from £171.6m for the same period in 2010.
Marks said competition from all major retailers, greatly diminished customer confidence, and the start of the government's austerity cuts played a part.
"At the full year we warned that the downturn was biting deeper than anyone had expected and predicted that challenging trading conditions would continue into 2012," he said. "Indeed, it is the worst I have seen in more than 40 years of retailing and, against this backdrop, the results are in line with our expectations."
He added that he did not see the situation improving over the next 12 months. "Looking ahead, we do not see signs of any real improvement in the economy and are planning accordingly to help customers as much as possible through this difficult period."