The 30-year-old has been training with City for the past 10 days and joins on a free transfer, having been released by Portsmouth at the end of January.
Head coach Sean O'Driscoll said: "He's got npower Championship experience and is technically very good.
"Brian was available and financially fits into the model going forwards.
"His personality will add something positive to the group."
Howard began his professional career at Swindon before spells at Barnsley, Sheffield United, Reading and Millwall.
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