Greenwood and Manchester City youngster Phil Foden allegedly invited girls into the team hotel following last Saturday’s 1-0 win over Iceland in the Nations League where they both made their Three Lions debuts.
The two players did not take part in training on Monday morning and Southgate confirmed later in his press conference that they have been sent home from team camp.
While Greenwood will miss Tuesday’s game against Denmark as a consequence, his club Manchester United have condemned his actions.
The 18-year-old has now come out with an apology and has promised to learn from his mistake.
"Having now had the chance to reflect on what's happened, I can only apologise to everyone for the embarrassment I have caused," Greenwood said in a statement posted to United's website.
"It was irresponsible of me to break the COVID-19 protocols which are in place to protect players, staff and the public.
"In particular, I want to apologise to Gareth Southgate, for letting him down, when he had shown great trust in me.
"Playing for England was one of the proudest moments in my life and I only have myself to blame for this huge mistake.
"I promise my family, the fans, Manchester United and England that this is a lesson I will learn from."
