A strong second half for the home side. Three penalties plus a try from Hudson Tonga'uiha killed the game off.
The Pirates didn't give up and pulled a late try back from Ryan Storer, but this was cancelled out by Jack Moates.
So defeat on the night and on aggregate secures London Welsh the Championship trophy.
Welsh are now expected to take legal action over the RFU's ruling that they are not eligible for promotion to the Premiership.
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It will be mild and cloudy today with patchy rain. Light rain will continue into tomorrow, turning heavier in the afternoon.
Bob Crampton has the Thursday weather.