Lee Bradbury scored twice to earn Portsmouth a point as he returned to haunt his former club and provide a temporary setback to City's automatic promotion hopes.
Joe Royle's side appeared to be strolling towards a comfortable victory before a crowd of 19,015 at Fratton Park as they took a two-goal advantage inside the opening 41 minutes.
Defender Spencer Prior headed them in front before striker Robert Taylor doubled the lead with a neat finish at the near post.
But Bradbury pulled a goal back on the stroke of half-time from the penalty spot after Taylor had handled Justin Edinburgh's hopeful ball into the area.
And the former City striker earned Pompey mathematical safety from relegation in the 84th minute when he headed home inside the left-hand post.
Bradbury endured a miserable spell at Maine Road after his £3.5million move to City nearly three years ago.
He struggled for form before being transferred to Crystal Palace for a cut-price fee.
But the former soldier returned to Portsmouth earlier this season and he showed he had lost none of his finishing prowess as he stole between two defenders and powered home Ceri Hughes' cross.
Bradbury could even have scored a winner in the dying seconds as he lobbed the ball over goalkeeper Nicky Weaver and shot towards goal but defender Richard Jobson somehow got his body in the way to block the effort.
Royle and his City players were left to regret throwing away a two-goal lead after they had taken control of the match with an impressive first-half display.
Prior was well placed to head home Gerard Wiekens' cross in the 26th minute at the near post and Taylor doubled the advantage 15 minutes later when he produced a neat back-flick to fool Portsmouth goalkeeper Russell Hoult at the near-post.
Bradbury's penalty restored hope for Portsmouth but City's leading scorer Shaun Goater should have restored the two-goal advantage in the 76th minute when he headed inexplicably wide from just six yards, and he was left to regret his miss as Bradbury pulled the scores level eight minutes later.