Millwall Under 23s slipped to sixth in the Professional Development League on Monday afternoon following a narrow 1-0 defeat to Cardiff City at Calmont Road.
Lloyd Humphries' first-half penalty separated the sides 27 minutes into the encounter, with chances few and far between on a soggy afternoon.
Despite an improved second half from The Lions, they were unable to find a breakthrough against the South Wales side and now sit seven points outside the promotion places in the division.
The game began evenly, with both teams looking to get directly in behind their opposing defences. The back lines however dominated the first quarter of the game and were able to see to any threat of a chance from either side. The visitors did find a momentary sniff of goal, as Ben Margetson headed wide.
It was shortly before the half-hour mark that the first shot in anger on target arrived, and with it came the opening goal. Some good build-up play from Cardiff saw the ball worked out to the left-hand side of a crowded penalty area, with Lions captain Harry Donovan bringing down his man in the process.
With the referee showing no hesitation in pointing to the spot, Bluebirds skipper Lloyd Humphries stepped up from the defence and struck a firm penalty past Ben Amos and into the right-hand corner.
The second half began in a livelier manner, with Kevin Nugent's Lions looking to push forward in search of an equaliser. George Alexander worked Cardiff goalkeeper Matt Hall into his first save, with a stooping header to connect with Junior Tiensia's ball from the left. Joel Bagan's flicked header inches past the post was the visitors' best opportunity of the half.
Millwall were to force a scramble for the ball in the six-yard box late on, as smart work from substitute Reuben Duncan down the right saw him find Alexander, whose improvised pass looked to find Joe Wicks. The midfielder's effort to pull the trigger was foiled by a committed Cardiff defence, who were eventually able to deal with the danger.
In a scrappy end to proceedings, The Bluebirds were able to see out the lead and take the three points back onto Welsh soil.
Millwall: Amos; Hanson, Debrah, Parker, Tiensia; Skalak (Mitchell 67'), Donovan (c), Wicks, Saunders-Henry (Harvey 78'); White (Duncan 46'), Alexander.
Subs not used: Sandford, Trialist.