Linfield slumped to a fourth successive league defeat under former Northern Ireland striker David Healy as Portadown sealed a 2-0 home victory.
Former Linfield defender Chris Casement scored the opening goal five minutes into the second half.
Ross Redman, who seconds earlier had cleared off the line from Andrew Waterworth, set up the score with a great run down the left wing.
Marcio Soares made it 2-0 as the match was entering stoppage time.
It was a breakaway goal, with Gary Twigg unselfishly squaring the ball for substitute Soares to score into an empty net.
Just before that Linfield had mounted pressure but home keeper David Miskelly made a number of good saves.
Linfield stay in fourth because the team immediately below them, Glenavon, were beaten 2-0 at Glentoran.
Pressure mounting on Healy
It is the first time Linfield have lost four league games in a row since March 1997.
And this latest defeat will place additional pressure on Northern Ireland's record scorer Healy who was only appointed Blues manager in mid-October when the they were joint top with Crusaders.
Now Linfield find themselves 12 points behind the defending champions after 18 matches of the league campaign.
Next week Linfield are at home to Coleraine who are up to second in the table after an unbeaten run of 11 Premiership matches.
What they said
Portadown assistant manager Kieran Harding: "The real turning point was the clearance off the line by Ross Redman who then got up the other end to set up the goal.
"He got to the bye-line to pull the ball back for what was an outstanding goal from our perspective.
"We had to defend for our lives and overall it was a great team performance."
Linfield manager David Healy: "I thought today we probably didn't get the rub of the green, but I am not one for excuses and we have lost another game of football.
"I have told the players to get their heads up because we have a big game on Tuesday against Carrick in the semi-final of the Shield.
"I have been working 100% every day to try to turn this club around and make it as successful as it has been before."