The young Bluebirds slipped to a damaging defeat in their Professional Development Under-18 League match against Sheffield Wednesday at Cardiff International Sports Campus.
Teenage midfield player Liam Shaw scored Wednesday’s winner after 72 minutes and Cardiff City slipped from top spot in the PDL South table to second.
Ipswich Town are the new leaders, while Leeds United and Wednesday are the top two in PDL North.
The Owls remain a point behind Leeds in the Under-18 North table, but have two games in hand with four left to play.
After a flowing Wednesday forward move involving Shaw and Lewis Farmer, Conor Grant was denied from inside the box by a stunning save from Bluebirds goalkeeper George Ratcliffe.
The Cardiff shot-stopper was alert from the resulting corner to tip a shot over the bar from Michael Ellery’s header.
Owls made the all-important breakthrough on 72 minutes. Grant’s clever flick-on sent Shaw through on goal and he calmly finished to clinch victory for the away team.
Cardiff City: George Ratcliffe, Taylor Jones, Ryan Kavanagh, Keenan Patten, Joel Bagan, Sam Bowen (capt), Ntazana Mayembe, Harry Pinchard, Siyabonga Ligendza, Dan Griffiths, Rubin Colwill (Eli King 59). Subs not used: Jordan Duffey (gk), Pritchard, Ashford, Smith.
Sheffield Wednesday: Render, Dawodu, Ellery, Waldock (Reaney 76), Thompson, Rice, Farmer, Shaw, Hammoud, Grant, Brandy (Hall 81).
Cardiff’s remaining Professional Development Under-18 League fixtures:
Wednesday 3 Sheffield United (away)
Saturday 6 v Crewe Alexandra (away, 11am)
Tuesday 9 Bolton Wanderers (away, 11am)
Saturday 13 v Burnley (home at Cardiff International Sports Campus, Leckwith, 11am)