Two tries from Jack Moates helped London Irish defeat Mont de Marsan and keep alive their hopes of reaching the Amlin Challenge Cup quarter-finals.
A young Exiles side led 10-0 at the break thanks to Moates' try and five points from Steven Shingler's boot.
The visitors gained a 17-0 lead when Matt Garvey kicked ahead and Moates touched down before Shingler converted.
Two tries from Simon Lucu, the second a length-of-the-field effort, was not enough to secure a late comeback.
London Irish's victory, their third in the competition, means they still have hope of pipping Gloucester into the last eight.
Although, for that to happen the Exiles must win their last match and hope Gloucester lose both their remaining matches, away at Bordeaux on Friday and at home to Mont de Marsan on 19 January.
Mont de Marsan: Duvallet, Durquet, Lucu, Mchedlidze, Beaulavon, Claverie, Magnaval, Correia, Bordes, Giudicelli, De Chaves, Murray, Brethous, Farre, Ricaud.
Replacements: Douvesy for Duvallet (69), Ryder for Beaulavon (62), Paquet for Bordes (6), Fiorini for Giudicelli (45), Lescure for Ricaud (60).
London Irish: Joseph, Watson, Shingler, Armitage, Yarde, Geraghty, Moates, Yanuyanutawa, Blaney, Aulika, Sandford, Low, Garvey, Gibson, Gray.
Replacements: Ojo for Watson (72), Gaston for Yarde (72), Evans for Low (75).
Referee: Marius Mitrea (Italy)