The FA is to launch an investigation into the actions of Phil Foden and Mason Greenwood while both were on England duty in Iceland.
Both players have been dropped from the England squad ahead of Tuesday's UEFA Nations League game with Denmark in Copenhagen.
Foden and Greenwood broke the team's COVID-19 rules.
Icelandic publication DV say the players invited women back to their hotel rooms after their 1-0 win over Iceland on Saturday.
"We had to decide very quickly they could not have any interaction with the team and could not travel to training," England manager Gareth Southgate told a press conference on Monday.
"Given the procedures we have to follow they will have to travel back to separately."
Manchester United said that they were "disappointed with the actions of Mason Greenwood", and that they'd be liaising with the FA.
Foden's club Manchester City released an even stronger statement, saying, "It is clear that Phil’s actions were totally inappropriate."
They added, "His behaviour not only directly contravenes strict guidelines related to Covid-19, but also falls well below the standard expected of a Manchester City player and England international.
"The club supports the FA regarding this incident, and officials from the Club are now in touch in relation to Phil’s enforced early return to the UK."
Neither player are required to quarantine upon their return to the UK, and neither are in action on the first weekend of the 2020/21 Premier League season.
The FA themselves released a statement on Monday evening, which read:
Whilst in Iceland, both Phil Foden and Mason Greenwood spent time outside of our private team area, which was a breach of our COVID-19 rules.
While they did not leave the team hotel, it was an unacceptable breach of our protocol. They have both apologised for their serious lack of judgement.
The FA will be launching a full investigation into the circumstances leading up to this breach. We have spoken to the Football Association of Iceland to offer our apologies for this incident and to ensure them that we are taking the appropriate steps.
We have also assured the Danish Football Union that all other players and staff members have been isolated within our group throughout this period.
Our immediate priority now is preparing the team for the match against Denmark tomorrow.