A 13th minute David Nugent strike was all that separated these two play-off chasers today, and Sven Goran-Eriksson's Foxes will be left to rue what could have been after wasting a series of chances as the visitors held on to claim all three points.
Eriksson made changes to the side that played out a convincing 3-0 win away at Scunthorpe United last weekend. Chelsea loanee Bruma regained his place in the starting eleven, bizarrely replacing last Saturday's goal scoring hero, Miguel Vitor.
The Foxes started brightly and began to show their attacking intentions early on. A well worked passing move between Wellens and Gallagher resulted in a cross that Yakubu headed inches wide.
However, once Portsmouth settled down, they looked a difficult side to break down and Leicester knew they were facing a tough afternoon against a side that had kept seven clean sheets in their last eight outings.
And, the Foxes faced an uphill struggle after David Nugent's 13th minute goal gave the visitors a lead to protect.
An inswinging corner from Lawrence was cleared by Sol Bamba after Ricardo had attempted to punch the ball away, but the ball fell to former England striker David Nugent who was able to side foot the ball past Ricardo to further tarnish the Portuguese 'keepers deteriorating reputation.
Sol Bamba nearly equalised for the home side immediately when some good work from Yakubu won the Foxes a corner. Gallagher swung the ball in but Bamba's strong header was saved superbly by Ashdown.
Nugent was again causing problems for the City defence and he could have doubled his tally for the afternoon when he was left unmarked to head home from a Lawrence corner. However this time the striker mistimed his jump and headed weakly into the ground.
Leicester began to impose pressure on the Pompey goal but just couldn't make the breakthrough.
An impressive passing move involving King and Wellens culminated in a wonderful lay off from Yakubu, but Gallagher's shot whistled wide of Ashdown's right hand post.
For all of Leicester's dominance, they trailed at the break as Portsmouth continued to look solid and composed in defence.
Sven Goran-Eriksson's men started the second half in much the same manner as they had finished the first period but, despite creating several chances, Portsmouth held firm.
On the hour mark, Eriksson replaced Yuki Abe with the pacey Lloyd Dyer in a bid to breathe life into the Foxes and the move nearly paid dividends as Dyer posed an instant threat.
Leicester's best chance of the match fell to Paul Gallagher. Bruma played a delicious diagonal ball to Gallagher, but the winger's shot was well blocked by the onrushing Ashdown.
The Foxes continued to push for an equaliser and were nearly punished on the counter attack.
Kitson and Lawrence linked up excellently around the penalty area as the City backline chased shadows and when the ball was played into the path of Ward a second Portsmouth goal looked imminent, but the midfielder fluffed his lines, sending the ball wide of Ricardo's gaping goal from six yards.
Portsmouth weren't made to pay for missing their chances though as the visitors survived several late scares - including a powerful goal-bound drive from Kyle Naughton that was tipped around Ashdown’s front post - to hold onto their lead and take all three points back to Fratton Park.
Although Leicester's second home loss on the bounce sees their poor run of late continue, the optimists amongst you will be pointing to the fact that teams above Leicester lost today and, the gap between City and a play-off place remains just five points.
With just eight matches to go, Eriksson and his men now have two weeks to reflect on what has been a disappointing month and hopefully prepare to challenge all the way for a place in the play-offs.