Gareth Southgate has praised the impact of the young players who have recently been called into the England squad, and says they are making it tough for him to decide his best team.
Since the World Cup in the summer, Southgate has added young players such as Joe Gomez, Ben Chilwell and Jadon Sancho to the squad that memorably reached the semi-finals.
In recent months, England have recorded a brilliant victory in Spain, beaten Switzerland and the USA, and earned a draw in Croatia.
"We've added depth to the squad with more young players emerging, and more competition for places," Southgate said.
"We've had a slight change of system, which has added something different. We've got some flexibility in how we can play.
"We're always improving and we haven't sat on our laurels. It's becoming increasingly difficult to pick a team and that's a really good sign. Young players have come through and done really well.
"The only way to avoid complacency and keep driving performances is to have that competition."
England play Croatia at Wembley on Sunday - live on Sky Sports - in a Nations League match that will decide which team progresses to the semi-finals.
It will be the third meeting between the two sides since July, and Southgate says England know what to expect from their opponents.
"We know their style of play and we know areas we think we can try to exploit. It's two really evenly-matched teams.
"We've got to make sure we back-up our last few performances, which have been really healthy, and finish the calendar year on a real high.
"We've proved ourselves to be two of the best teams in the world this year.
"We've got huge respect for them and their mentality, but we've also got the belief that we're improving.
"We've got great motivation ahead of tomorrow's match."