The Canadian international striker is set to make a move back to the Championship after an unsuccessful stint in the Bundesliga.
Jackson, who joined Braunschweig in the summer from Premier League side Norwich, was limited to just nine appearances in the first-half of the Bundesliga season.
Den manager Ian Halloway will be hoping that Jackson can replicate his old goal scoring form in the Championship with Norwich, where in the 2010-11 season he helped the Canaries clinch promotion.
Millwall sits just above the relegation zone with 26 points from 27 games.
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