Spanish goalkeeper Iker Casillas reported for the first day of pre-season with Porto on Monday, two months after suffering a heart attack, but won't train with the team for a while.

"Back to work. First day," the 38-year-old tweeted alongside a photo that showed him arriving, smiling, with team-mates at Porto's Olival training centre.

The former Real Madrid and Spain goalkeeper underwent emergency surgery on May 1 after suffering a heart attack during training. He left the hospital five days later, admitting that his professional future was uncertain.

"For the moment he cannot train," his agent Carlo Cutropia told the Porto O Jogo newspaper.

"He certainly wants will be there on the first day at Olival to be with his teammates. Someday he will come back, but he can't train yet. The doctors will say if he needs to wait one, two or three months. We don't know. We'll see."

Casillas played 167 times for Spain, winning two Euros and one World Cup, and more than 700 games for Real, where he won the Champions League three times.

He joined Porto in 2015 and extended his contract by one year in March. Club president Jorge Pinto da Costa has said he wants Casillas to be playing for Porto at 40.