"Nonnatus House: Urban demolition," said the letter from Greater London Council in bright red capital letters, followed by some ominous words: "This is a notice to inform you that Nonnatus House is currently included within the designated demolition area for Poplar."
After the letter arrived at the start of series nine, Sister Julienne (Jenny Agutter) kept this news a secret from everyone except Sister Hilda (Fenella Woolgar), warning her to "tell no one". But now she has finally admitted the truth to her team: Nonnatus is on course to be pulled down in the next two years.
So is this really the end of Nonnatus House?
"Well, no spoilers but it's very much a part of the journey that Nonnatus House is going on," Heidi Thomas told RadioTimes.com at the Radio Times Covers Party.
"And it reflects what was going on in the East End at that time, and indeed what happened to the original order of nuns who were based there and whose work inspired our series.
"I think it would not feel very realistic if they weren't under some sort of threat. Their premises are rented, we know they've already had to move once before because of an unexploded bomb that then exploded.
"And now everything is being torn down around them. We see that on screen, and it would be very strange if it didn't impact on them, physically and emotionally."
The nuns and midwives were previously forced to move at the start of series three, when the original Nonnatus House was earmarked for demolition – a process sped up by the discovery (and controlled detonation) of a World War Two bomb. But they've been in their present HQ ever since.
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Call the Midwife is currently based at Longcross Film Studios in Surrey, where the production team have built a massive set including the exterior of Nonnatus House, the square, shops, bridge, and Hendy Street allotments. Various Nonnatus interiors are also filmed on set here.
So does the on-screen threat to Nonnatus mean that Call the Midwife is moving to a new set?
"Trick question, so I can't say," Thomas told us. "Because we do have, we're about to start filming series ten and then there's series 11. So who knows what will happen before the end!"
As for the prospect of Nonnatus being demolished, "I think all of the midwives are feeling unsettled by it," Nurse Trixie actress Helen George said.
"It's their sense of community, it's their workplace, but it's their family as well. And a lot of these women have found themselves there with - and they're there for a reason as well, you know, they've gone to find a new family outside of their own biological family. There's a reason for that, perhaps. So it's emotionally a massive thing to feel that that sense of community amongst themselves is going to potentially crumble.
"Should fans be worried? I mean you don't know! Let's just wait and see. It might be intense next year!"
Fortunately for fans, whatever the outcome for Nonnatus House, Call the Midwife itself is already commissioned for another two series. Bring on series 10 and 11...
Call the Midwife continues on Sundays on BBC One at 8pm