The 19-year-old will travel out next week to join up with Keith Downing’s side, completing a superb debut season for full back, who played 40 times for the first team.
Hatters boss Nathan Jones said: “It’s 100 per cent justified. It’s a credit to the process we have here at the club, it’s not just a credit to the youth team or to me or anything, it’s a collective thing.
“The process we have got, we believe in young players, so we try and recruit good young players and then coach them to make them better, to give them an opportunity.
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“There is a pathway at this football club, that if they are good enough they will be given an opportunity and JJ has taken that opportunity with both hands.
“There’s a lot of credit there. It’s not a credit to me. Yes, I gave him an opportunity and developed him further, but it’s a collective thing.
“From his recruitment, to the youth team, the coaching he had, the actual pathway he had at Luton Town – a club that thinks like that. It’s a wonderful thing for JJ.”
The Young Lions head to Murcia for six days next week as they look forward to their trip to the UEFA European U19 Championships Finals in Georgia in July.
Justin will join a group of 23 players for the trip, who will also have further training camps back at St George’s Park before the tournament begins on July 2.
England secured their space in the finals after picking up three wins from three games against Norway, Spain and Belarus in the elite qualifying round in March.
They take on Bulgaria, Holland and Germany, with Justin earning his first call-up alongside Arsenal’s Josh DaSilva, in a squad that includes former Town youngster Jay Dasilva as well.
Goalkeepers: Will Henry (Swindon Town), Aaron Ramsdale (AFC Bournemouth), Nathan Trott (West Ham United).
Defenders: Trevoh Chalobah (Chelsea), Jay Dasilva (Chelsea), Tom Edwards (Stoke City), Cameron Humphreys (Manchester City), Darnell Johnson (Leicester City), James Justin (Luton Town), Reece Oxford (West Ham United), Easah Suliman (Aston Villa), Dujon Sterling (Chelsea).
Midfielders: Josh Dasilva (Arsenal), Andre Dozzell (Ipswich Town), Tayo Edun (Fulham), Jacob Maddox (Chelsea), Mason Mount (Chelsea), Ryan Sessegnon (Fulham)
Forwards: Ben Brereton (Nottingham Forest), Isaac Buckley-Ricketts (Manchester City), Marcus Edwards (Tottenham Hotspur), Lukas Nmecha (Manchester City), Chris Willock (Arsenal)