Pompey were unable to halt their losing run, despite a much improved performance at Peterborough.
Joe Gallen's side were beaten 1-0 to drop to 10th in the League One standings, with an unfortunate Rasmus Nicolaisen own-goal settling the contest.
The Blues were the much better side after the break and some dangerous Lee Brown and Ronan Curtis deliveries caused Posh problems, but they could not find an equaliser.
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