Southampton wasted the chance to return to the top of the Championship as goals from David Nugent and Matt Mills gave Leicester victory at St Mary's.
Saints were two down after 26 minutes, former Portsmouth striker Nugent opening the scoring by cutting in from the left to curl home a 20-yard shot.
Former Southampton defender Mills added a second with a header from six yards.
Dean Hammond headed Southampton's best chance across goal, while at the other end Paul Konchesky struck a post.
It was a second successive home defeat for Southampton, who
Their next home league game on 31 January takes on added importance as it is against third-placed Cardiff, who are currently only one point behind them.
For Leicester, managed by former Saints boss Nigel Pearson, it was the completion of a double over the south coast side, following their
Both sides had early chances at St Mary's, last week's FA Cup hat-trick hero Jermaine Beckford firing wide from 12 yards inside a minute for Leicester, and David Connolly also off target for the Saints.
A 20-yard Connolly strike was comfortably saved by Kasper Schmeichel and, after Nugent's impressive ninth goal of the season, the Saints struggled to find a way back into the match.
When Sean St. Ledger picked up the ball on the left and delivered an inch-perfect cross for Mills to head home his first goal for Leicester it proved too much for the home side to recover from.
Jack Cork fired over when well placed four minutes before the break after good work from Morgan Schneiderlin but, soon after the resumption, Nugent came close to a third for the visitors, sending another curling effort just wide.
Schmeichel kept out Hammond's far-post header, while Konchesky was unfortunate to see his free-kick come back off the woodwork and roll to safety.
In the closing stages Saints subtitute Lee Holmes fired wide with a sorry effort that summed up the home team's evening.
Southampton manager Nigel Adkins:
"We were up against it tonight. As I have said on many occasions, when things aren't going right that's when you find out about people and we have found out about all of us tonight in certain ways.
"Emotions are quite high at the moment but we will reflect on it in the cold light of day. We need to work very hard to keep driving this club forward.
"We have to grind out performances and tonight wasn't good enough, we know that.
"We will reflect on the performance and reflect on the result, we still have a long way to go and we have to work very hard and stick together."
Leicester boss Nigel Pearson:
"I am very pleased for the players. They have put a lot in to the game tonight and they have got what they deserved.
"It is all well and good having talented players but whatever level you play at you must always work harder than the opposition and the work ethic really was exceptional.
"This is a long way to come for us all and for the supporters especially it is a great result.
"They made a tremendous noise and backed the players from the start right until the finish."