Arsenal midfielder Jack Wilshere said he “could have made a real impact” for England at the World Cup.
The 26-year-old said he should be on the plane to Russia as he has felt “fit, sharp and strong all season”.
“It goes without saying, I’m incredibly disappointed,” said Wilshere.
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The Arsenal midfielder, who has had an injury-plagued career, made 38 appearances for Arsenal in the 2017-18 season, including 12 Premier League starts.
He won the last of his 34 England caps in the Euro 2016 defeat by Iceland.
“I believe I should be in the squad,” he added.
“However, I have to respect the manager’s decision and would like to wish the whole squad all the very best for the tournament.
“I will always be an England fan and will be supporting the boys with the rest of the nation.”
Southgate defended his decision to omit Wilshere.
“Jack Wilshere has of course been in our thoughts,” Southgate said on Thursday.
“We selected him in March, but he hasn’t played a game for us.
“We’ve been pleased with the players who played in November and March. The only midfielder we added is Fabian Delph, who had a fantastic season with the champions.
“Jack had a good spell but was maybe not as effective towards the end of the season. We’re very open-minded in terms of what we want.”
World Cup build-up
Watford midfielder Nathaniel Chalobah, 23, and Manchester City goalkeeper Angus Gunn, 22, will link up with the England squad as training players.
Southgate has also confirmed that Bournemouth midfielder Lewis Cook, who is one of the stand-by squad players, will join the under-21s for the annual Toulon tournament in France which starts on 26 May.
England start their World Cup build-up next week without players from FA Cup finalists Manchester United or Chelsea, who will have a week’s rest, and Liverpool, who are preparing for the Champions League final.