Millwall suffered a major blow to their hopes of automatic promotion as they slipped to their fourth home defeat of the season against London rivals Wimbledon.
The Dons, who enhanced their own chances of a play-off spot in the process, took all three points thanks to Kevin Cooper's 15th-minute strike.
But it was Millwall who started brightly, despite playing into a fierce wind and a driving snow storm.
With just five minutes gone top scorer Richard Sadlier almost presented Paul Ifill with a chance but the winger was muscled out of it by desperate defending.
Five minutes later, the snow had disappeared and the sun was shining but the home side were still making the running.
Belgian winger Christophe Kinet swung in a corner which Sean Dyche got up to, but Wimbledon goalkeeper Kelvin Davis punched clear in the nick of time.
Moments later another Kinet corner caused mayhem but the Lions were unable to take advantage with both Stuart Nethercott and Sadlier just missing out.
It proved a costly miss when the visitors promptly went up the other end and scored.
Millwall's defence watched absent-mindedly as Cooper took aim from nearly 30 yards out and buried his low shot in the bottom corner past the in-form Tony Warner.
Millwall came on strongly just after the half hour when Sadlier crossed for Kinet to toe-poke goalwards from close range, but again Davis denied him.
The home side's frustration built with David Livermore earning a caution in first-half stoppage time for a reckless challenge on Damien Francis.
The visitors almost added a second just after the break when Joel McAnuff's cross eluded Matt Lawrence, but Cooper's first touch gave the right-back the chance to make amends with a vital block.
But the home side should have levelled matters on 55 minutes when another Kinet corner caused confusion and Dyche and Neil Harris somehow failed to find the net from close range.
With six minutes left, Millwall manager Mark McGhee replaced midfielder Livermore with Ipswich's on-loan striker Richard Naylor, but his side almost conceded another immediately.
Warner managed to spill an innocuous ball, but David Connolly's instant strike found only the side netting.
With time ticking down Millwall almost snatched an equaliser when Ifill found Kinet in acres of space on the left but the Belgian's cross was intercepted by Jermaine Darlington with Naylor poised.