The young Scarlets lead by 6pts to nil midway through the first half but conceeded the games only try close to half time and with Cardiff fly half Nick Macleod adding the conversion and a penalty it was the Arms Park youngsters who lead by 10pts to 6 at the interval.
Cardiff are virtually assured of the league title ahead of next weekends visit to Bridgend to complete the formalities of the title success and will now play Neath in the cup final. The game will be played at a neutral venue and the date is yet to be decided.
Llanelli started by far the brighter of the two sides but despite their flow of possession failed to convert their advantage into points other than two penalties from fly half Caerlon Thomas, while Cardiff attacked from deep to create their only clear cut scoring opportunity which wing Elgan Jones rounded off wide out.
Fira fly half Macleod converted superbly and his penalty on the stroke of half time gave the home side a lead at the interval.
The visitors responded well after the restart with scrum half Mike Phillips tackled short of the try line after a quickly taken penalty had taken play 50 metres up field and fullback Barry Davies created problems for the home side with a kick upfield before Cardiff regrouped. The home side moved play to within sight of the posts and Macleod calmly added his second penalty of the evening to give his side a crucial 7-point.
Despite the disruption through injury to their front row, the Stradey Park youngsters dominated the set pieces and looked threatening in attack as the crowd of over 500 waited in expectation of a visitors score.
However, Llanelli tried in vain to secure the vital score losing possession at crucial times and despite their endeavour and resolve, the visitors failed to break the final line of defense as Cardiff held onto victory and the chance for their unique double of titles.
Meet the members of the WRU's inaugural Youth Board - after an online selection process which saw a host of high calibre candidates submit video applications the WRU has made its first 15 appointments.The board is made up of individuals aged between 16 and 21, and will meet for the first time at Principality Stadium on Monday 15th February.
The Wales Women's team headed to Dare Valley Country Park on a team bonding exercise as the entire squad got together for the very first time in preparation for this season's Six Nations championship which kicks off on Saturday with a clash against defending champions Ireland, in Donnybrook.
Wales Head Coach Warren Gatland discusses his team selection for the opening round of the RBS 6 Nations where he leads his side against Ireland, on Sunday. As WRU TV's Luke Brodley discovers, Gatland is fully aware of the challenges the defending champions pose
WRUTV caught up with Wales Women captain Rachel Taylor at the RBS 6 Nations launch and how a good crowd really helps boost the girls performance. Tickets for all three Women's home games at The Gnoll, Neath are available from www.wru.wales/tickets