Cardiff booked a Twickenham showdown with Gloucester in the final of the EDF Energy Cup after edging a tough encounter with Northampton 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.

Cardiff edged into an early lead when Dylan Hartley was penalised straight from the kick-off and Ben Blair slotted the simple kick.

Hartley, in typically fiery mood, frequently overstepped the mark to receive a warning from referee George Clancy after conceding three early penalties.

But the England hooker was also a central figure in Northampton's powerful rearguard and he twice won turnover ball to relieve the pressure.

Blair missed a second penalty shot at goal and Stephen Myler also wasted an early chance to put Northampton on the scoreboard after Molitika had been isolated.

Eventually, Cardiff's pressure paid off. Hartley missed his man in the lineout and the Blues ran hard and straight through Molitika and the increasingly influential Wales centre Jamie Roberts.

Leigh Halfpenny had a dart for the line but Northampton left fringes unguarded and Spice darted over from the base of a ruck.

Northampton made a blistering start to the second period. Neil Best's bulldozing run through the middle set them on their way before the ball was shifted wide for Ansboro, who held off Halfpenny's tackle to dive for the line and score in the corner.

Halfpenny engineered a positive response from Cardiff with two electric breaks but the Saints' scramble defence held firm again and Tom Shanklin was bundled into touch just short of the line.

Northampton defended a messy lineout but were then penalised at a scrum, offering Blair a chance to extend Cardiff's lead.

He missed a third straight kick but Barry Everitt, on for Myler on the hour, then continued the poor goal-kicking spectacle by missing a chance to draw Northampton level.

Cardiff continued to test Northampton's defence with a powerful run from Roberts and earned a penalty which Blair slotted to push them six points clear and seal a date at Twickenham.