Goalkeeper Joe Hart and midfielder Jack Wilshere won't be incorporated, as detailed by BBC Sport on Tuesday.
Britain begin their Group G crusade against Tunisia on 18 June, before recreations against Panama on 24 June and Belgium on 28 June.
Britain will play Nigeria on 2 June and Costa Rica on 7 June in home friendlies previously leaving for Russia.
Southgate can't call up Liverpool match Alex Oxlade-Chamberlain and Joe Gomez, who have been discounted through damage, alongside Swansea protector Alfie Mawson.
After two friendlies in March, Southgate said that "19 or 20" of his squad had been settled on, yet midfielders Adam Lallana, Jonjo Shelvey and Ruben Loftus-Cheek, alongside youthful Liverpool appropriate back Trent Alexander-Arnold, are among those in dispute for the periphery places.
Those picked will get together next Monday - separated from players engaged with the FA Cup and Champions League finals.
Britain could have named a bigger temporary squad - up to 30 - before trimming down to 23 in time for Fifa's due date of 4 June however previous midfielder Frank Lampard trusts Southgate's approach is the correct one.
Britain's key World Cup dates
16 MaySquad declared
2 Junev Nigeria (benevolent)
7 JuneV Costa Rica (benevolent)
12 JuneSquad leaves for Russia
18 Junev Tunisia (opening gathering diversion)
24 Junev Panama (second gathering amusement)
28 Junev Belgium (last gathering amusement)