The defending champions were Swansea City, who won the 2016–17 competition.
AFC Wimbledon, Fulham, Huddersfield Town, Reading, West Ham United, did not return after playing 4 seasons in the competition.
Bury and Dagenham & Redbridge will participate for the first time in the competition this year.
A qualifying round was required to finalise the 32 teams that would enter the Group Stage.
Note: Flags indicate national team as defined under FIFA eligibility rules; some limited exceptions apply.