Real Madrid hotshot Cristiano Ronaldo has left the door open for a stunning return to English champions Manchester United.

"I really miss English football; some of the best years of my career were at United and everyone knows that," said the former World Footballer of the Year, who in 2009 moved from the Old Trafford outfit to Madrid for £80 million (S$155 million).

"The club is still in my heart. In the future, you never know but for now, I'm enjoying myself in Spain."

The 28-year-old forward was speaking to reporters during a visit to Crest Secondary on Wednesday afternoon to lend his support to the Singapore Olympic Foundation-Peter Lim Scholarship, which helps needy young athletes achieve their sporting goals.

Ronaldo was accompanied by supermodel girlfriend Irina Shayk during the 30-minute session at the school in Jurong East.

The couple, who had spent the past few days on holiday in Bali, later tweeted a picture of themselves at Universal Studios.