There was no turning away when Claire Foy and Matt Smith's chemistry steamed up our TVs in The Crown.
Viewers watched as the co-stars breathed passion into the Queen and Prince Philip's stilted relationship – and it turns out the stars may not have been acting!
With reports the 37-year-old Dr Who star has split from his girlfriend of five years Lily James, 30, rumours are ablaze he has found comfort in the arms of his co-star Claire, 35.
Despite their time on the show being over, friends say Claire and Matt couldn't handle being without their "on-screen spouse" and the couple have been spending countless hours together working on the two-person play Lungs in London over autumn, for which they've had rave reviews, and hanging out at their local pub in North London.
"There was chemistry between them on the first day they auditioned in 2016," says a friend.
"No one's surprised one bit. They've got the same sense of humour and answer each other's questions. You get that when people star together over a long time, but this is something else. They almost sizzle – you hold your breath when they look into each other's eyes. Incredible chemistry."
No wonder a double date in 2017 with their then-spouses Lily and Stephen Moore, 42, turned into a disaster.
"Claire and Matt got on so well they tried to include their other halves, but that was the writing on the wall," adds the insider.
"Seeing the chemistry in person is undeniable. No one who knows them is surprised by the fact they've hooked up. They have so much in common and can talk independent plays and scripts until the sun comes up."
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As for heartbroken Lily, who recently returned from a "post-traumatic" healing holiday with friends in South Africa, sources say she's way off the mark to suggest Matt and Claire cheated, with her post patting a cheetah.
"Neither of them are the type to stray, despite what Lily may say," adds the insider.
"The wonderful thing is it's a romance born from friendship, naturally progressing into something more serious. Claire's super-cautious about moving too fast because she has her daughter [Ivy, four] to consider, but we haven't seen either of them this happy in years!"