Pompey's winless run was extended to four games after they were beaten by Swindon at the County Ground.
Brett Pitman's brace gave the hosts a commanding lead, but Ronan Curtis reduced the deficit for Danny Cowley's side before George Byers and Andy Cannon both went close to finding an equaliser.
But the Blues were unable to find another goal and Tyler Smith wrapped up a 3-1 win for the Robins in stoppage-time.
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