London Welsh held off a spirited fightback from Italian side Cavalieri Prato to earn their first European match in dramatic fashion.
Tries from Gordon Ross and Joe Ajuwa had helped Welsh to a 26-17 lead with 20 minutes remaining.
Prato made a host of changes and fought back to take the lead thanks to tries from replacement duo Niccolo Tempestini and Giacomo Bernini.
Welsh trailed by a point with four minutes to play but fought deep into Italian territory and were awarded with a crucial late penalty.
Full-back Alex Davies made no mistake under pressure to secure Welsh a foothold in the Challenge Cup and help erase memories of last week's disappointing performance against Stade Francais.
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