Lee Molyneux denied Pompey a place in the top three tonight.
The Morecambe defender struck with eight minutes left to earn his side a 1-1 draw at Fratton Park.
That cancelled out Kal Naismith’s eighth goal of the season seconds after the sub came on in the 63rd minute.
With 71 away fans travelling from Morecambe, that sent the vast majority of the 15,592 crowd home frustrated.
Pompey were dominant in the first half but the game lost its impetus after the break.
That changed after Naismith’s introduction and the home side looked comfortable until Molyneux’s sucker punch.
It was a first half summed up by the half-time stats of 17 Pompey shots to Morecambe’s three.
Gary Roberts, Carl Baker, Gareth Evans and Eoin Doyle all had chances in a frenetic opening.
But Pompey were indebted to David Forde who pulled off a fine save to deny Aaron Wildig a certain goal 13 minutes before the break.
Lee Molyneux made another last-ditch stop to deny Roberts just before the interval as Pompey couldn’t make the breakthrough.
The second half saw a drop in impetus from Pompey.
Cook sent on Conor Chaplin in place of Carl Baker. His second change had a huge impact, though, as Naismith broke the deadlock seconds after replacing Kyle Bennett.
Naismith’s first intervention was to advance from Michael Doyle’s pass and bury a 25 yarder past Barry Roche.
The pace off the game meandered from there with Cook’s side appearing comfortable.
But Molyneux had other thoughts as he struck with an angled drive with seven minutes remaining.
Two stoppage-time scrambles in the Morecambe goal came to nothing as the game finished all square.
The consolation for Pompey was knowing they were now two points off Carlisle in third with a game in hand.