A statement on the League Two club's website revealed the 24-year-old is suspended pending a disciplinary hearing following the results of a routine drugs test after their match against Dagenham on Boxing Day.
Marshall has consequently missed the Bees' last two matches.
Barnet say in mitigation that Marshall claims to have "unwittingly taken a banned substance in a supplement".
The club added they would not be making any further comment at this stage.
Meanwhile, defender Anwar Uddin, 30, has had his Underhill contract terminated by mutual consent.
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