Zlatan Ibrahimovic is threatening to sue a former Swedish track and field coach for speculating that he used performance enhancing drugs during his time at Juventus.

Ulf Karlsson, who was the head coach of Sweden's national athletics team from 2001 to 2004, said during a sports conference this week that he thinks Ibrahimovic, 34, doped in order to bulk up so quickly at Juve.

"Zlatan gained 10 kilos of muscle in six months at Juventus. That is impossible in such a short time," Karlsson was quoted as saying by local newspaper Nya Wermlands-Tidning.

Mino Raiola, Ibrahimovic's agent, said his client plans to take legal action against Karlsson over the claims.

"We are going to sue him. I'm not an expert on Swedish law, but we have a Swedish lawyer," Raiola told Swedish newspaper Expressen. "There will always be haters. Maybe he is a former athlete who didn't make it. Maybe he is jealous."