Irishman Padraig Amond scored the winner as Newport delivered a huge FA Cup shock at a rocking Rodney Parade to beat Leicester 2-1 and pile the pressure on Foxes manager Claude Puel.
County bridged a gap of 74 places on the league ladder after Jamille Matt's early header was cancelled out eight minutes from time, Rachid Ghezzal's thunderous leveller having seemingly spared the Premier League visitors.
But three minutes later Marc Albrighton handled Vashon Neufville's cross on the corner of the box and Carlow native Amond stroked home the resulting penalty.
It was no less than Newport, 12th in Sky Bet League Two and with only one win in all competitions since the end of November, deserved and rekindled memories of their cup heroics from last season.
On those occasions Leeds were beaten and Tottenham held at home before Michael Flynn's side bowed out of the competition in a Wembley replay.
Leicester had been warned but Puel, who was sharply criticised for naming a weakened side when the Foxes suffered a Carabao Cup exit to makeshift Manchester City opponents last month, still chose to make seven changes from the New Year's Day win at Everton.
England internationals Jamie Vardy, Harry Maguire and Ben Chilwell were among the absentees, and Puel can expect more brickbats to come his way on the back of Leicester losing their last chance for silverware this season.
The pattern of a breathless first half was set inside the opening six minutes when Newport goalkeeper Joe Day denied Ghezzal and Shinji Okazaki.
Ghezzal's strong burst ended with another goal attempt but Newport went ahead with their first attack of the game.
Robbie Willmott produced some terrific old-fashioned wing play to beat Christian Fuchs and his cross was met by Matt, the striker claiming his 14th goal of the season off a post.
The goal maintained Matt's record of scoring in every round of the competition after previous success against the Met Police and Wrexham.
Danny Ward had to move quickly to stop Matty Dolan from doubling Newport's lead from a 25-yard free-kick, but there was no chance to pause for breath as Leicester attacked at the other end.
Albrighton appeared destined to equalise until Tyler Forbes intervened with a brave block, and from the resulting corner Okazaki planted a free header wide from inside the six-yard box.
Leicester constantly looked dangerous in the final third, but County were more intent on doubling their advantage than sitting back on their lead. All of which made for a rip-roaring affair.
Dolan's low drive tested Ward and a better Matt touch from Amond's cross would have added to Leicester's embarrassment.
Okazaki passed up another opportunity to level after Fuchs surged down the left, his shot striking Mickey Demetriou, and his profligacy saw him replaced by James Maddison at half-time.
Maddison was needed to provide extra guile and County came under increasing pressure as Day held Kelechi Iheanacho's header and Albrighton struck the crossbar with a swerving effort from 25 yards.
A rare mistake from Demetriou allowed Iheanacho another sight of goal, but the striker sent his first-time shot wide when he might have been better served by taking a touch.
Newport were forced to defend deep as the second half wore on but centre-backs Demetriou and Fraser Franks were immense.
Leicester looked as if they would survive when Ghezzal smashed home from the edge of the box, but the Welsh side had other ideas and Amond kept his cool from 12 yards to send them into round four.