The Tottenham full-back, who 10 months ago put England ahead in the match against Croatia in Moscow, was one of four players released when Southgate was forced to hand in his official final 23 ahead of next week’s semi-final against Holland in Guimaraes.
By his own admission, Trippier has had an indifferent season back at Spurs after he was one of the revelations of last summer’s tournament.
And with Trent Alexander-Arnold continuing to develop and Kyle Walker being used as a full-back again rather than as part of a back three, it was Trippier’s name that was surplus to requirements.
“Of course, Kieran Trippier has had such a big impact,” Southgate said. “He had such a brilliant World Cup. He has been a really valuable member of our squad and has a great attitude with us, real passion in playing for England.
“But Trent and Kyle have finished the season so strongly. So it is a position where we have got real competition for places. That is as hard a decision as we have had to make.”
Spurs team-mate Harry Winks also failed to make the cut having never had a chance to prove his worth after groin surgery in April. While some of the England squad have been training at St George’s Park, he is busy preparing for the Champions League final.
But while club manager Mauricio Pochettino is encouraged by his progress, it was that lack of time on the training pitch for his country which ultimately counted again him.
“Harry’s a tough one because he has not played for a couple of months and we don’t get to see him in training,” Southgate said.
“He is back training with Spurs but we wouldn’t see him until two days before the game. So when we had other midfielders, that was the only thinking behind that decision.”
Despite being part of the reduced group at St George’s Park, Southampton pair Nathan Redmond and James Ward-Prowse are the other two to miss out – although they will continue to train with the group as it continues to come together bit-by-bit.
He said: “What is nice is you get another burst of energy as each group arrives – so today [Monday], Raheem Sterling, Kyle Walker, John Stones and Jadon Sancho came in for the first time.
“Again, the training shifts up another level and everybody is excited to be back together and we will get that again next week when the rest of the team report as well.”
The final players will report in following the two European finals between English clubs, with Southgate entitled to make late medical switches should any unforeseen injuries occur.