Mason Greenwood and Phil Foden have both apologised after Gareth Southgate confirmed the pair will not travel with the England squad from Iceland to Denmark for breaching UEFA's coronavirus protocols.
The FA has launched a "full investigation" into an incident which saw Greenwood and Foden break quarantine rules while at their team hotel in Reykjavik on Sunday night.
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Southgate said the duo broke coronavirus secure bubble guidelines and have been sent back to England, while the remaining squad are now in Denmark ahead of Tuesday night's Nations League encounter - which will go ahead as scheduled after talks between UEFA and FA officials.
Foden and Greenwood allegedly met two women in a different part of the hotel away from the bio-secure area the England team were staying.
Icelandic police are continuing their investigation and will interview the two women and have not ruled out possible prosecutions against them.
Following statements from their respective clubs, Manchester City midfielder Foden, 20, and Manchester United forward Greenwood, 18, both released personal apologies.
"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 on Tuesday in a statement on 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."
On Monday, Foden issued a lengthy apology on Instagram: "Following the story which has emerged today regarding my actions in Iceland, I want to make a full apology.
"I apologise to Gareth Southgate, to my England team mates, to the staff, supporters and also to my club and my family.
"When I was called up by Gareth for these games, my first reaction was that of immense pride. To pull on that shirt for the senior team in my England debut was an incredible privilege.
"I am a young player with a lot to learn, but I am aware of the huge responsibility I have in representing Manchester City and England at this level. On this occasion I made a poor decision and my behaviour didn't meet the standards expected of me.
"I breached COVID-19 protocols put in place to protect myself and my England colleagues. As a consequence I will now miss the opportunity to travel to Denmark with the squad, and that hurts. I will learn a valuable lesson from this error in judgement and I wish Gareth and the team good luck this week."
Foden will not have to quarantine after returning to the UK and will be tested for Covid-19 before he is able to resume training with City.
Greenwood will also be tested before he can train with United, while the club will await the findings of an FA investigation before deciding what action to take against the striker.
Southgate to 'support' Foden and Greenwood
Iceland's police force confirmed to Sky Sports News the players have each been fined 250,000 Icelandic Krona (£1,360) for the rule breach, which must be paid by the individuals themselves and not by the FA or their clubs.
After labelling Foden and Greenwood's actions as "naive", Southgate insisted he would support the pair, although it remains unclear whether he will consider them for next month's Nations League matches.
"I think I'm very conscious that these two boys are going to walk into something that's going to be very intense, and very difficult for them at their age to deal with," Southgate said.
"Whether people like it or not I've got to try and support them and I've got to try and not add to how difficult their situation is going to be. That said, I will be very clear with them on my feelings and how they need to respond."
Southgate said the breach had occurred outside of the part of the hotel the FA has booked for the England squad.
"Nothing has happened in the areas we occupy in the team hotel," he insisted.
"We are still getting to the depths of all the information because it was only brought to my attention only a couple of hours before training, I am still getting to grips with some of the detail.
"What is clear is that there was a breach of the coronavirus guidelines. We spent such a long time getting those measures in place, the whole squad followed that together.
"We have had four tests in the period we have been together. Our staff have been incredible in keeping that secure and tight. In that respect we have no alternative but to do what we are doing."
FA to launch full investigation into breach
UEFA has accepted the version of events provided by Southgate and England officials, and also that the rest of the England squad remains bio-secure.
As a result, European football's governing body concluded there is no reason for Tuesday night's Nations League game to be compromised.
An FA spokesperson said: "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."
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The pair were the only players absent as Southgate took his final tactical session at the Laugardalsvollur Stadium in Reykjavik before Tuesday's game.
Both players made their full international debuts in Saturday's 1-0 Nations League over Iceland.
Southgate will have a 21-man squad to choose from for the game in Copenhagen, which is live on Sky Sports Football from 7pm on Tuesday, with Kyle Walker suspended after receiving a red card in Reykjavik.
Denmark manager: It's not a good situation
Denmark remain hopeful the England game will go ahead but will speak to UEFA ahead of Tuesday's meeting.
Manager Kasper Hjulmand says they will accept whatever outcome is decided.
"It's not a good situation," Hjulmand said. "The Danish FA will take care of this and look into it. The protocols are very strict and we will accept whatever UEFA comes up with. If they think it is a good idea to keep going then of course we will. We hope we can play this game.
"It's very important that we stay within the protocols that we have been given. It's about football and the survival of football, being able to play is very important.
"There's been a lot going on so that we can actually play these matches so we have to be focused."
Man Utd and Man City condemn Greenwood and Foden behaviour
Manchester United say they are "disappointed with the actions of Mason Greenwood over this situation" and it is understood they will await the findings of an FA investigation before deciding what action to take against the 18-year-old.
Greenwood will be tested for COVID-19 before he returns to training at Carrington.
Meanwhile, Manchester City say Foden's "inappropriate" behaviour falls below the club's standards and the club support the FA regarding the incident.
"It is clear that Phil's actions were totally inappropriate," City said in a statement.
"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.
"We have no further comment to make on this matter."
How to watch Nations League on Sky Sports
You will be able to watch all 162 Nations League matches live on Sky Sports - plus the semi-finals, third-place play-off and final. Games will be shown across our main channels and via the red button.
If you're out and about, you can follow all of those Home Nations games on the Sky Sports website and app, where we'll have minute-by-minute updates and show you the goals as they go in.
We'll also have highlights of every game involving the Home Nations and the best of the other Nations League fixtures available too.