With the Three Lions dealing with issues such as injuries and European final absentees, the Football Association confirmed a larger provisional squad would be announced on Tuesday rather than the definitive group.
Speaking about his decision to name a 33-man squad, Southgate explained: "I think our situation is more complicated than any other country at the moment.
"We have known for a while that we have of this 33, 12 players still to play in European finals this week so we were always going to need some additional players for the first part of the camp.
"Add to that a number of injuries...so we felt that more time is going to help us make better decisions."
There are first senior call-ups for Brighton centre-back White and Everton defender Godfrey, while Sheffield United goalkeeper Ramsdale joins uncapped Sam Johnstone in the squad as Nick Pope misses out through injury.
Eric Dier misses out, but Alexander-Arnold returns to the group alongside fellow right-backs Reece James, Kieran Trippier and Kyle Walker. Bukayo Saka, Ollie Watkins and 17-year-old Jude Bellingham are among the others to get the nod.