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.
Welsh players and management joined Deputy Minister for Sport Ken Skates at the Pierhead Building, Cardiff Bay earlier this week to reveal the Union's pioneering school-club hub initiative has been so successful the project will now almost double the scale of the original project.
The Scarlets, Newport Gwent Dragons, Cardiff Blues and the Ospreys look ahead to Judgement Day III which sees the Blues take on the Ospreys (2.30) and the Dragons tackle the Scarlets (4.45pm) at the Millennium Stadium tomorrow afternoon.
Ystradgynlais and Ynysddu played out a titanic struggle at Taffs Well in the SSE SWALEC Bowl semi-final. At the end of the game neither team could be separated after a 15-15 stalemate but Ystradgynlais go through on try count.
Blues strike first in JD III battle - Newport Gwent Dragons lock Andrew Coombs, WRU referee Dylan Llyr Jones and Cardiff Blues hooker Matthew Rees decide dressing room allocations for Judgement Day III
Wales Under 18 captain Calum Haggett has received a conditional offer to read Biomedical Science at St Anne's College, Oxford. WRU chairman Gareth Davies went to meet Calum and Coleg y Cymoedd Principal Judith Evans to wish him luck in his exams and pass on some of his personal experience.
Dodgy tattoos, carthorses, Avatar and soft chins are revealed by Wales Sevens players Tom Williams and Sam Cross as they profile the Wales Sevens squad taking part in Hong Kong this weekend in the latest round of the HSBC Sevens World Series.