Colwill earned his first senior England call-up on Thursday after nailing down a starting spot at Stamford Bridge under Mauricio Pochettino.
The 20-year-old shone on loan at Brighton last term and trained with the senior England squad in June before helping the Under-21s win the European Championship without conceding a goal.
Colwill was forced off with a minor knock during Chelsea’s 2-1 Carabao Cup win over AFC Wimbledon on Wednesday, but he is fit to face Nottingham Forest on Saturday and will then join up with the England squad for games against Ukraine and Scotland.
“He has the ability to defend the box well like any young defender,” said England manager Southgate. “He showed a really good temperament in the League last year, and in the summer with the Under-21s he was outstanding.
“With Levi, Marc [Guehi and Fik [Fikayo Tomori], there is an element of looking to the future, because there’s going to be a change in that area of the field.
“The more we can find out about them, the better. Levi has a real maturity about his calmness with the ball. He’s got good physical attributes. He’s got speed and the ability to defend the box well.”