England will be in League A; Wales, Northern Ireland and the Republic of Ireland are in League B; while Scotland are in League C.
Teams in the top tier, League A, will compete to become "UEFA Nations League champions" with the winners of the four groups going through into a "Final Four" play-off competition, with two semis and a final, to be crowned the overall Nations League winners.
It will also provide Ireland with a second chance to qualify for Euro 2020, if needed, with the four highest-ranked non-qualifying League B teams entering play-offs for one finals place in March 2020.
Germany, Portugal, Belgium, Spain, France, England, Switzerland, Italy, Poland, Iceland, Croatia and the Netherlands make up League A.
Split into four leagues - A, B, C and D - the inaugural competition will see Uefa's 55 member associations play group stage matches from September 2018 and the finals from 5-9 June 2019.
The victor of each of the four groups will be drawn to face each other in a Euro 2020 play-off.
"How teams perform in the Nations League will be used to determine which pots they will be placed in for future European qualifiers for the Euros and the World Cup", the Sun said.
Northern Ireland are also in League B, alongside a number of Ireland's recent opponents, Austria, Sweden and Bosnia and Herzegovina. However, if a whole league does not have enough teams for a play-off, which is conceivable in League A, then the places will be filled by the best-ranked teams from the league below.
The four group winners are promoted to League A, with the four sides that finish bottom relegated to League C for the next competition, to be played in 2020.
League C contains Hungary, Romania, Scotland, Slovenia, Greece, Serbia, Albania, Norway, Montenegro, Israel, Bulgaria, Finland, Cyprus, Estonia and Lithuania.
For example, with League D guaranteed a representative, Azerbaijan, Macedonia, Belarus and Latvia are the favourites to battle it out over a series of one-off contests to join, but not necessarily be competitive in, the finals jamboree.
This continues accordingly until each of the groups have four teams in the play-offs.
THE Uefa Nations League is set to get underway next year with England and Wales the leading home nations.
The qualification for Euro 2020 will now now run from March to November 2019, with Dublin hosting the Group draw on 2 December 2018.