Einar Sparboe Lysnes cautioned in a statement on Thursday that in the last few months the situation had reached a crisis point.
"We have several times discovered that there were no free spaces, and so simply had to let them back out," he told VG newspaper. "As a result new criminal offences take place that otherwise would not have happened. Our ability to prevent crime and ensure safety is reduced."
Lysnes' warning followed a complaint earlier this month from the head of the police in Vestfold county, near Oslo, who proposed freeing up space by converting old army camps in far-northern Norway into emergency prisons for foreign criminals.
Lysnes said that on Thursday, the day he made the problem public, he had been forced to release a 20-year-old repeat offender, who had been charged this time with three serious thefts, six burglaries and fraud.
Earlier in the autumn, he was forced to release a car thief who only a few weeks later went on to carry out further car thefts, until he was finally stopped for driving a vehicle when drunk.
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Jorid Midtlyng, who heads the prison services in Norway's northern region, admitted that prisons in the region were at full capacity.
"We apologize for the situation, but we are jam-packed," he said. "We do everything we can… yet it does occasionally happen that we can't offer places."