The publisher would have posted a pre-tax profit in the year to 31 December 2008, but was dragged into the red by what it termed impairment losses of £55.7m.
The company did not reveal what this charge - typically a special, one-off charge taken to write down an asset with an overstated value - was for.
However, many media companies, including ITV and News Corporation, have written down the value of some assets following the impact of the recession on their businesses.
NatMags' turnover was down to £344.3m in the period, from £356.94m a year earlier. Revenue from UK operations was £293.7m, down from £309.4m a year earlier.
NatMags declined to comment further.
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