The striker is officially on standby for the championships in Portugal but will nevertheless travel with the squad for a training camp in Sardinia before taking part in the Manchester international tournament at the start of June.
England play two matches in that pre-Euro 2004 event - Japan on June 1, Iceland on June 5, both at the City of Manchester Stadium.
Only 23 players will travel to Portugal however with Michael Owen, Emile Heskey, Wayne Rooney and Darius Vassell the four strikers selected.
"Jermain was very close," confirmed Eriksson this afternoon. "He's not one of the 23 but he's number 24. He's coming with us to Sardinia and will take part in the tournament in Manchester as well.
"If something happens in the squad he might come in."
Jermain Defoe has just missed out on Euro 2004 - coach Sven Goran Eriksson has named him as the England squad's number 24 this summer.