A comment under a WorldBuilding.SE answer to the question What's the quickest way to tell if most of the people on Earth have disappeared? refers to the BBC Newsbeat article Trident: What are the letters of last resort? describing the instructions for the Trident nuclear submarines what to do in case Britain has been 'wiped out' by a nuclear attack:

When are the letters opened?

"The submarine has to make a judgement that the UK has been hit by a nuclear strike," according to Prof Seligman.

"The commander does that by trying to make contact with the UK via Naval Command or listening out for radio signals."

It's thought one of the key tests is whether the Today programme on BBC Radio 4 is still broadcasting.

If all the checks fail, the commander will go to the safe, remove the letter and find out what the orders are.

Is the claim correct that listening out for BBC radio signals (or more specifically the Today program) is one of the official means of determining that the UK has been hit by a large nuclear attack?

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    not that the claim is made in terms of - 'it is thought'
    – user43646
    Apr 25 '18 at 11:38
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    yeah, I'm just pointing out that there's a good chance you'll not find anything definitive when the claim itself is not a definitive claim. Nothing wrong with the question :)
    – user43646
    Apr 25 '18 at 11:54
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    Your title asks if the silence of Radio 4 is THE trigger. But the article says "one of the key tests...". In other words the commander should consider the absence of a regularly broadcast live radio show as evidence (and probably consider other radio stations too) but if "Today" goes off the air it's not going to trigger an attack if everything else is normal. Apr 25 '18 at 14:19
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    I would assume that at least most of the official tests for determining when to launch nuclear weapons are kept secret to avoid them being spoofed or attacked. I don't think this is a claim that can be definitively answered in regards to current tests, but there's always a chance it was a semi-official test at some point.
    – Giter
    Apr 25 '18 at 15:21
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    It's also important that "The submarine has to make a judgement". In other words there isn't a simple checklist of things that the commander follows. Which is what we would expect. Apr 25 '18 at 16:43

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