Scarlets fly-half Stephen Jones slotted an injury-time penalty to reduce the arrears, while Biarritz had their star flanker Serge Betsen sin-binned and lost wing Jimmy Marlu through injury.

Llanelli, bidding to reach their third Heineken Cup semi-final in five seasons, fielded a full-strength side.

Scarlets boss Gareth Jenkins made one change from the team that secured a last-eight spot through victory at Northampton two months ago, replacing lock Adam Jones with experienced Wales international Chris Wyatt.

Biarritz included five players who featured during France’s Grand Slam-winning RBS 6 Nations campaign – full-back Nicolas Brusque, fly-half Julien Peyrelongue, scrum-half Dimitri Yachvili, number eight Thomas Lievremont and flanker Betsen.

Both sides appeared riddled by nerves during an error-strewn opening, with poor handling and wayward kicking dominating the first 10 minutes.

Jones’ first kick at goal – a 48-metre effort – hit the post below the crossbar and Biarritz responded by trying to free speedster Marlu but they also missed a chance to go ahead when Yachvili’s angled penalty attempt drifted wide.

The Scarlets were immediately stretched by a flowing Biarritz attack, and a scrappy first quarter ended with both sides still at the sparring stage.

Much of the entertainment came from Biarritz’s travelling brass band, as neither team could find a rhythm, especially in attack. Defences dominated, and the game desperately needed some inspiration.

Biarritz almost broke through 10 minutes before the break. Yachvili put Couzinet clear, and quality support work from his second-row partner Herve Manent and prop Denis Avril created a chance for skipper Lievremont.

Llanelli though, had sufficient numbers to quell the attack, and number eight Scott Quinnell’s ball-carrying power out of defence put Biarritz in reverse gear.

But as the interval approached, Biarritz built a solid set-piece platform and drove the Scarlets over their own line.

Referee Chris White could not establish whether or not one of the Biarritz forwards had grounded possession, and after a delay, the video official agreed there was too much doubt.

Biarritz were not finished, and they demolished a Llanelli scrum before flanker Ovidiu Tonita was held up by a brilliant tackle from Scarlets wing Garan Evans.

Llanelli battled vainly to hold out, yet the visitors struck on the stroke of half-time when Couzinet crashed over from close range and secured a slender interval advantage.