Sky Sports are reporting that the Blues are interested in signing striker Gavin Massey.
The 24-year-old, who scored eight goals for Leyton Orient as they were relegated from League Two last season, would be available on a free transfer.
He has exercised an option to leave the Brisbane Road club following their drop into the National League – despite signing a three-year deal in 2016.
According to Sky, Pompey will face competition from former boss Paul Cook, now at Wigan, and Bradford for Massey’s services.
The forward scored for the O’s in their 2-1 defeat to Pompey at Fratton back in January.
He also scored twice as Colchester beat the Blues 3-2 in League One in 2013.