Ulster Ravens paid a heavy price for David McGuigan's first-half yellow card to see their hopes of reaching the quarter-finals severely dented at Old Deer Park.
The Ravens conceded three tries in a six-minute spell in the flanker's absence, and must now beat both Neath and Moseley in their last two Pool C fixtures if they are to have any chance of progressing.
Patrick Jackson's early penalty for the visitors was cancelled out by Dominic Shabbo's try, before the game swung decisively in London Welsh's favour when McGuigan was sent to the bin. The home side immediately made their numerical superiority tell when former London Irish centre Shabbo crossed for his second try, and three minutes later scrum-half Rob Lewis slipped inside Conor Carey's tackle to dive over.
Jackson replied with his second penalty, but Josh Drauniniu weaved through several tackles and Lewis again was there to finish from close range. The Ravens were offered an early second-half lifeline when Welsh prop Will Johnson saw yellow and a minute later Mark McCrea sent Tommy Seymour in for a try but Lee Beach's try killed off any lingering hopes of a Ravens comeback.
London Welsh -- A Thomas; L Gibson, D Shabbo, P Mackey, J Drauniniu; W Luveniyali, R Lewis; S Pittman, D George, W Johnson; D Williams, M Corker; B Russell, M Hills, L Beach. Reps: E Taione for Russell (35), S Nelson for George (46), N Runciman for R Lewis (58), G Ross for Luveniyali (58), J Lewis for Drauniniu (58), L Ward for Johnson (60). Yellow card: Johnson.
Ulster ravens -- T Seymour; M McCrea, M Allen, L Marshall, C Gilroy; P Jackson, M Heaney; C Carey, N Annett, A Whitten; R Caldwell, I Henderson; S Lavella, D McGuigan, N McComb. Reps: N O'Connor for Jackson (44), J Shiels for Marshall (44), B McIlroy for Heaney (44), P Pritchard for Caldwell (44), K McCall for Carey (56), J Burns for N Annett (61), R Hackney for McGuigan (61). Yellow card: McGuigan (25).
Ref -- Neil Hennesy