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.
WRU TV were on hand at the launch of a major new commercial deal with WRU and Mercedes-Benz South Wales (part of the Sinclair Group), which sees the family company become an official supplier to the Welsh Rugby Union
On the back of the recent Women's Rugby World Cup, the WRU has launched a revitalised Women's and Girls domestic seasonal structure, and the future is looking bright.
Go to www.wru.co.uk/tryourgame for more information
Roger Lewis talks to WRU TV at the announcement of a new £60 million Rugby Services Agreement (RSA) which provides a new and positive contractual relationship between the WRU and its four Regions for the next six years has been agreed.
The Welsh Rugby Union has announced a major new sponsorship deal with Gullivers Sports Travel which will make the UK's leading, and longest established, multi-sport tour operator, an Official Supplier for the rest of the decade.
Having taken part in seven World Cups as a player and coach, Liza Burgess hopes to go out in style this afternoon when Wales take on Australia in the Play-off for seventh and eighth place in the Women's Rugby World Cup in Paris.
WRU Head of Rugby Josh Lewsey met with volunteers across the game earlier this week, championing their role in the sport. He was joined by Wales captain Sam Warburton for part of the day as volunteers took part in a 'Try Out' event for the 2015 Rugby World Cup