"Since the quarterly release in August 2011 the global economic uncertainty and pressure on freight rates have persisted," Torm A/S said in a statement.
Chief Executive Jacob Meldgaard said in the statement that especially freight rates for larger vessels had been below expectations in the second half of 2011.
"It has been a challenging market for a prolonged period, but Torm remains confident that in the longer term the product tanker segment will benefit from improved market fundamentals," Meldgaard said.
The warning came ahead of nine-month results due on Nov. 17.
Shares in Torm plunged 23 percent by 1244 GMT, returning to levels visited in mid-October.