Instruments used to show how many radioactive particles are released have been giving wrong readings since 2004, and in fact levels were three to four times greater than previously believed.
Staff have only just noticed the errors and have informed the Swedish Radiation Protection Authority. It says the increased amounts are still within safety guidelines, but says it is worrying that it has taken so long for the problem to come to light.
On Wednesday the Swedish Nuclear Power Inspectorate banned Forsmark from restarting one of its reactors which were taken off-line earlier this month because of problems with a rubber seal. The second one can not be started again until the plant provides a thorough safety report.
Last month the Inspectorate filed a police complaint against the station over the handling of an emergency shutdown last July, and the Chief Executive of the plant resigned last week.