The Portuguese club said the 37-year-old goalkeeper was "well" after his "acute myocardial infarction". 

"Iker Casillas suffered an acute myocardial infarction during Wednesday morning's training session," a Porto statement read.

"The session was promptly interrupted to provide assistance to the FC Porto goalkeeper, who is currently at Hospital CUF Porto. 

"Casillas is well, stable and his heart problem [is] solved."

A number of messages have been sent to the stopper, including one from fellow Spaniard and 17-time tennis Grand Slam champion Rafael Nadal. 

Casillas captained Spain to a first World Cup final triumph in 2010 and was awarded the Golden Glove for his performances at the tournament. 

He also led his national side to memorable successes at the UEFA European Championships in 2008 and 2012, contributing to his 167 international caps – a Spanish record.

Móstoles-born Casillas has spent the majority of a glittering domestic career with Real Madrid, winning three UEFA Champions League trophies and five La Liga titles over 510 appearances.

He moved to FC Porto in 2015.