Manchester City begin the defence of their Champions League title this week as the group stage gets under way.

Pep Guardiola's side are joined by Arsenal, Manchester United and Newcastle from the Premier League, with the latter making their return after a 20-year absence.

Scottish champions Celtic are playing in the group stage for the second successive season.

Here's all the key information about the 2023-24 Champions League.

Who has the toughest Champions League group?

Newcastle have been drawn in a tricky Group F alongside French champions Paris St-Germain, Italian giants AC Milan and former Champions League winners Borussia Dortmund.

City will fancy their chances of qualifying from Group G with little difficulty as they face RB Leipzig, Red Star Belgrade and Young Boys.

Celtic have what appears on paper to be a favourable group featuring Feyenoord, Atletico Madrid and Lazio.

The obvious concern for United are German giants Bayern Munich. But, with FC Copenhagen and Galatasaray also in Group A, United will feel they have enough to go through.

Arsenal are up against Europa League winners Sevilla, PSV Eindhoven and Lens.

Who are the big names missing?

For years, the prospect of seeing Cristiano Ronaldo and Lionel Messi on European club football's greatest stage was a given, but neither will feature in this season's Champions League.

Ronaldo was one of the first big names to move to Saudi Arabia - he plays for Al Nassr - while Messi is in the United States with Inter Miami.

They are not the only two absentees, Brazil forward Neymar and France striker Karim Benzema having joined Ronaldo in the Saudi Pro League. They are with Al Hilal and Al-Ittihad respectively.

We will still get to see some of the biggest names of the present and future, with France striker Kylian Mbappe at PSG and City goal machine Erling Haaland looking to break more records.

Lamine Yamal is also causing plenty of excitement. The 16-year-old this year became Barcelona's youngest player of all time as well as the youngest debutant and scorer for Spain.

When are the Champions League games played?

Matchday one: 19-20 September.

Matchday two: 3-4 October.

Matchday three: 24-25 October.

Matchday four: 7-8 November.

Matchday five: 28-29 November.

Matchday six: 12-13 December.

Last 16: 13-14, 20-21 February 2024 and 5-6, 12-13 March.

Quarter-finals: 9-10, 16-17 April.

Semi-finals: 30 April, 1 May, 7-8 May.

Final: 1 June.

Where is the Champions League final?

The final this season will be held at Wembley.

Who are the Champions League favourites?

City are favourites to win the Champions League, ahead of Real Madrid and Bayern Munich, according to simulations based on Gracenote's Euro Club Index.

City have a 37% chance of victory, Real 13% and Bayern 11%, and there is a 19% chance that a new name will be on the trophy.

City, Real, Bayern and Barcelona each have at least a 90% chance of reaching the knockout stage.

When is the Champions League format changing?

This will be the last season of the existing format - 32 teams in the group phase, split up into eight groups of four, and the top two going to the last 16 - with widespread changes coming in for 2024-25.

The new format will feature 36 sides, with two of the four extra places awarded to countries whose clubs have performed the best in the previous campaign.