Best was unexpectedly taken ill during the World Cup last autumn and was rushed to hospital in Bordeaux amid fears he had suffered a stroke.

An irregular heart rhythm was diagnosed as the source of the problem and while the Ulster front row made a rapid recovery, he has been forced to undergo constant monitoring since.

Best has not been troubled by any further episodes or suffered any long-term effects, but doctors have urged against training to the level of intensity required by a professional rugby player.

The 30-year-old said: "It's with great sadness that I am forced to retire from the game at this stage in my career.

"However, I have no regrets and feel immensely proud and privileged to have represented both Ulster and Ireland for the past nine years."

The condition has not been attributed to Best's participation in rugby, and the popular former Ulster captain bows out with a fond memories.

He continued: "I'm grateful to have had the opportunity to work with coaches, management and players that I have had the utmost respect for.

"I would like to thank the Ulster and Ireland supporters and my friends and family for their loyalty and support throughout my career and during this difficult time.

"I would also like to take this opportunity to thank the Irish Rugby Football Union and their medical team for the extremely high standard of care that I have received.

"Finally, having grown up as a supporter of Ulster and Irish rugby, I would like to wish the players and management of both squads all the very best for the rest of the season and the coming years.

"I will continue to support and follow their future successes with great interest."

The Irish camp was shaken when news of Best's admission to hospital filtered through in the build-up to the final pool game against Argentina, the latest setback of disastrous month in France.

Approachable and honest, Best was held in high regard by his team-mates - and news of his recovery was greeted with enormous relief. 

He made three appearances during the World Cup and was a regular on the bench for Ireland, supplying back-up to first-choice props John Hayes and Marcus Horan.

He won the first of his 23 caps as a replacement against Tonga in 2003 and went on to participate in the World Cup later that year.

With senior players rested for last summer's tour to Argentina, Best was named captain and led Ireland in both Tests.

Ireland coach Eddie O'Sullivan said: "Simon has been an integral member of the Ireland squad since 2003.

"He was an incredibly talented and hard-working player in every aspect of his involvement in the game and a model professional.

"In addition to that, he was and still is extremely popular with all the players - and it is very unfortunate that his career has been cut short by this condition when he still had so much more to offer Irish Rugby.

"I would like to wish Simon and his family the best of luck for the future."