This follows the provisional clearance on 8 September by the Competition Commission of DS Smith's 170m acquisition of Linpac Containers (PrintWeek, 11 March).
Chairman of the inquiry group, Peter Freeman, said that the merger would not lead to a substantial lessening of competition.
However, he said a number of customers had expressed concerns about the level of competitive behaviour in the sheet market.
"We (the Competition Commission) believe that further investigation by the OFT would be justified," he said.
DS Smith is providing information to the OFT as part of the investigation into the sheet market.