Cardiff booked a Twickenham showdown with Gloucester in the final of the EDF Energy Cup after edging a tough encounter with Northampton 11-5 at the Ricoh Arena.
The Blues extended their unbeaten record against English clubs to 11 matches with a try from Bristol-bound scrum-half Jason Spice and six points from the boot of full-back Ben Blair.
Cardiff dominated the first half to open an 8-0 lead and although Northampton hit back quickly with a try from Joe Ansbro, they could not overhaul the Blues.
Cardiff's victory ensured an Anglo-Welsh flavour to the final for a fourth consecutive year. Wasps against the Scarlets in 2006 was followed by two Leicester-Ospreys encounters.
It was a deserved victory for the Blues, who bossed the first half with Maama Molitika's powerful running and Nicky Robinson's inch-perfect tactical kicking keeping Northampton pinned deep in their own territory.
Cardiff enjoyed nearly 70% of possession and territory and Northampton did not win any ball in Cardiff's 22 for the whole first half.
But the Saints defended resolutely and it took until the 36th minute for Blues scrum-half Spice to find a chink in the armour.
Andy Farrell admits the process of selecting England's 50-man training squad ahead of the World Cup was rigorous - but the backs and defence coach also revealed how tough it was to leave some stars out.