Nanterre - Former Springbok flyhalf Pat Lambie has opened up on his decision to retire from professional rugby.

The news, which was confirmed on Saturday, was down to post-concussion problems.

Lambie, a 28-year-old who has played 56 times for South Africa, announced his decision earlier in the day in an interview with French sports daily L'Equipe.

The flyhalf had been expected to return at the end of January after eight months on the sidelines, but had decided to end his career following numerous concussions and "persistent post-concussion symptoms", he told L'Equipe.

Lambie said he had already contemplated retirement before arriving in France after three concussions in South Africa, "two of them serious."  After the last, in May 2017, he did not play for six months.

He suffered a further concussion at Oyonnax in December 2017. The symptoms, he said, "lasted about 40 days".

"I was able to play again at the end of February (2018), I felt OK but I was not at my best," he said. 

He appeared in the European Cup final in May but limped off with a ligament injury as Racing lost to Leinster. 

He said that as his knee recovered the concussion symptoms grew worse.

He described "trembling legs," headaches, migraines and eye irritations.

"I have not been able to do any weight sessions in the last two and a half months because I had a terrible headache."

"I followed the advice of two neurologists," he said.

"They advised me to stop rugby."

Lambie was at La Defense Arena to see his team-mates beat Scarlets and win their pool in the European Champions Cup on Saturday. 

His decision to retire came as "a shock" to team-mate Dimitri Szarzewski.

"I immediately imagined myself in his place," said former France hooker Szarzewski, who is 35, adding that when Lambie arrived "we knew he was a bit fragile."

Fullback Brice Dulin said Lambie had off-field developments to focus on.

"Pat is the same age as me, it's weird. But it's a decision of life, he will be a dad very soon," Dulin said, after Racing advanced to the knockout rounds of a competition in which they have lost two finals in the last three years

"I just hope that in a few months we will have a nice tribute to him."