Meanwhile, it has also now been confirmed that Eddie Marsan (Back to Black) is joining the show, playing Charlie's therapist Geoff.
Author of the graphic novels on which Heartstopper is based and the show's creator, Alice Oseman, said in a statement: "I am incredibly excited to be welcoming three new cast members to the Heartstopper family."
She continued: "Hayley Atwell joins the Nelson family as Nick’s aunt Diane, who takes Nick on a summer holiday to Menorca and has some tough advice to impart about his relationship.
"Eddie Marsan will be portraying a beloved character from the graphic novels: Geoff, Charlie’s wise and straight-talking therapist.
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"And lastly, Jonathan Bailey will be making a cameo appearance as Jack Maddox, an Instagram-famous classicist and celebrity crush of Charlie.
"It was an absolute joy to see Hayley, Eddie and Jonathan’s talent alongside our existing cast, and I can’t wait for the Heartstopper fans to meet these new characters."
Alongside the announcement, Netflix also released a short teaser clip showing Bailey in character, which you can watch below.
The official synopsis for Heartstopper season 3 says: "Charlie would like to tell Nick that he loves him. Nick also has something important to say to Charlie.
"As the summer holiday ends and the months race on, the friends begin to realise that the school year will come with both its joys and its challenges.
"As they learn more about each other and their relationships, plan social events and parties and start thinking about university choices, everyone must learn to lean on those they love when life doesn't go to plan."
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One star of previous seasons who won't be returning this time around is Olivia Colman, who played Nick's mum Sarah.
Colman told Forbes that she "couldn’t do number three" because of scheduling, adding: "I feel awful about that. I feel like I was part of one of the most beautiful things I’ve ever been part of."
Heartstopper season 3 will be coming to Netflix this October, and seasons 1-2 are available to stream now. Sign up for Netflix from £4.99 a month. Netflix is also available on Sky Glass and Virgin Media Stream.
Check out more of our Drama coverage or visit our TV Guide and Streaming Guide to find out what's on. For more from the biggest stars in TV, listen to The Radio Times Podcast.
