Marseille was due to be the venue before covid restrictions were tightened in France.
Twickenham will host the European Champions Cup and Challenge Cup Finals next month.
Ulster are in the running to reach the Challenge Cup decider, which will be held at 8pm on Friday 21st of May.
The Heineken Champions Cup final, which could involve Leinster, will be staged a day later, on Saturday 22nd of May at 4.45pm.
ANNOUNCEMENT 📣— Heineken Champions Cup (@ChampionsCup) April 30, 2021
The iconic Twickenham Stadium will host the 2021 #HeinekenChampionsCup final with a limited number of fans permitted to attend 🤩
Details ➡️ https://t.co/naHHmlJQUA
Take a look back the glorious winning moments from the previous finals held at Twickenham 👇 pic.twitter.com/GrJB8zDuh1
Up to 10 thousand fans will be allowed in to see both games.
Marseille was due to showcase both finals, but it will be the host city for the 2022 deciders.