London Welsh recorded a dramatic 29-20 victory over Leeds Carnegie to set up a showdown with Bristol for a place in next season's Aviva Premiership.
Leeds had held a seven-point advantage after the first leg of their Championship semi-final and extended that advantage to 13 with just 12 minutes left to play at the Kassam Stadium on Sunday.
But Seb Steggman went over for a converted try in the 68th minute and when Oli Stedman crossed three minutes later the Exiles led 26-20 on the day and trailed by just a single point overall.
Gordon Ross then came back to haunt his former club, the ex-Leeds fly-half landing a penalty with two minutes to play to complete a sensational recovery by London Welsh.
Justin Burnell's men will now face Bristol in the two-legged final after they beat Rotherham 22-11 on Saturday to secure a 39-25 aggregate win.
The winners will replace Worcester in the top-flight after the Warriors ended the campaign bottom of the Premiership table.