To borrow one of the British Board of Film Classification’s carefully calibrated alerts, the next time you visit the cinema there is a “moderate threat” of an onscreen plot spoiler before the film, supplied by ... the British Board of Film Classification.

Anyone settling down to the Dardenne brothers’ Two Days, One Night since it went on general release will have found the frisson of anticipation as the houselights dim shattered by the matter-of-fact onscreen revelation that the film contains a “suicide attempt”. Er, spoiler alert?

Thanks to a laudable enough drive to “protect children and vulnerable viewers from unsuitable or potentially harmful content”, the BBFC has expanded its brief from forewarning to foretelling, by way of the supplementary “consumer advice” on what’s known in the trade as the “black card” – the traditional title deed bearing the relevant certificate in its familiar roundel that precedes the main feature. A similarly vexing fate befell Frank in May: viewers were preemptively advised of a “suicide scene”. Likewise before Paweł Pawlikowski’s 12A-certificate meditation on Polish identity, Ida. (The difference between a “scene” and an “attempt” is difficult to divine.)

Welcome to BBFC Insight – or “BBFCinsight” as it is unprettily amalgamated to impress people from the 90s – a crowd-empowering initiative launched to provide “clear and objective information” about the ratings it hands down, so that you and I can “sit down, relax and enjoy the film”. Which may be news to those drawn to Two Days, One Night by its rave reviews or intriguing trailer, which shows Marion Cotillard’s factory worker methodically trying to convince co-workers to vote for her reinstatement. The “suicide attempt” is not advertised in its publicity, and only a very inconsiderate critic would give it away, but thanks to BBFC mission creep, it’s not a case of if but when she tries to top herself.

Neil Alcock, a contributor to Empire magazine, raised the alarm on his film blog The Incredible Suit after seeing Two Days, One Night at Curzon Soho in London. He wrote: “I’d be fascinated to know exactly how many people, after arranging a trip to the cinema, buying their tickets and popcorn and taking their seats, made the decision to get up and leave with seconds to go before the film starts based entirely on the information given on the black card.”

The BBFC addressed his complaint, reminding him that “the specific details of the event – ie exactly who the suicide attempt concerned, where it occurred in the film and other details – are not mentioned.” This is a fudge. I contacted the director of Frank, Lenny Abrahamson, to gauge his feelings about his own BBFC giveaway and it transpires that he wasn’t aware of it. “Most definitely a spoiler,” he said. “Something which strongly affects the experience of watching the film, particularly as the suicide marks a big tonal shift.” For my money, there’s a big difference between flagging up vagaries such as “strong bloody violence” or “crude language” and announcing a key plot driver.

David Baldwin, assistant manager of the UK’s oldest working cinema, the Electric in Birmingham, said he and his staff were also surprised by the spoiler when they showed Two Days, One Night. “When the rating appeared on screen, somebody next to me whispered, ‘Suicide attempt? Well, now we know she’s going to try and kill herself.’ Clearly it marred their enjoyment of the film. Perhaps it could have been phrased ‘suicide themes’ rather than ‘attempt’.”

He pointed to another example of a BBFC spoiler, the American documentary Finding Vivian Maier in which an unknown Chicago photographer’s work was discovered after her death. “The ratings card noted ‘infrequent child abuse references’, which slightly ruined the impact of later revelations in what you thought was just going to be a documentary about a lady who took photographs.”

The bone of contention that links all these recent casualties of too much information is the certificate itself. In Vivian Maier’s case, it was a 12A (“unlikely to be suitable for children under the age of 12”). Without the child abuse references, it might have been rated PG (“Parents may wish to check the film before they let their children watch it”) or, more likely, U (“almost nothing unsuitable for children”).

When I contacted the BBFC, I was directed to their guidelines, most recently updated in February 2014. They state: “Portrayals of potentially dangerous behaviour (especially relating to hanging, suicide and self-harm) which children and young people may potentially copy, will be cut if a higher classification is not appropriate.” And, if the offending content is not cut, leading to a more forbidding rating, it becomes a “key classification issue”, which the board is now bound to publicly specify.

Assistant director of the BBFC David Austin said: “The suicide attempt in Two Days, One Night was key to its 15 classification and is therefore mentioned in the BBFCinsight. Although some members of the public may believe the BBFCinsight to be a spoiler in this case, it should be taken into account how much information is already available about a film via trailers and media reviews in the lead up to its release.” The implication seems to be: get over yourself.

On the whole, it’s difficult to fault the BBFC’s user-friendly website, which provides all the information a concerned consumer might need, including a detailed breakdown of every “key certification issue” in obsessively prosaic detail. In Two Days, One Night “a man aggressively directs a use of ‘bitch’ at a woman”, there’s “milder bad language, including uses of ‘damn’, ‘asshole’, ‘pissed’, ‘bullshit’ and ‘shit’” and “some moments of violence, including a couple of fights in which men push and shove at each other”. However, this insight is optional. You have to click on a logo to reveal it. There is no such opt-out when you’re sitting in the cinema.

Austin said that the board’s guidelines are based on consultation with more than 10,000 punters. “The public do consider imitable behaviour including suicide and self-harm to be a key classification issue, and therefore it is mentioned, when relevant.” The most recent research found that parents were “extremely open about how raising mentally stable children could not be taken for granted any more”.

Through social media, I was able to hear a first-hand account of the benefits of the BBFC’s suicide warning before Two Days, One Night from a 32-year-old avid filmgoer from London called Sam (not his real name). He describes himself as “someone with experience of suicidal thoughts” who received help a year ago and is now in “a good place”. He says: “When I encounter stories about people committing suicide, because I have first-hand experience of that kind of pain, I can almost empathise ‘too much’. Simply put: being forewarned allows you to prepare your safety net.

“When I take pleasure from a film, it’s from the way the story is being told, the acting, the cinematography, the editing. Your experience of an unknown plot is part of the pleasure of watching a film for the first time, but not an essential part.” He admits to being “a bit surprised” by the warning before Two Days, One Night, “but when the suicide came I realised I was grateful for being ready for it”.

You have to respect his position, which will no doubt validate what BBFC Insight is all about, no matter how isolated a case this may or may not be. However, accusations of nanny-statism will never be far away when the black card before current 12A family comedy What We Did on Our Holiday warns of “discriminatory language” and the U-rated Canadian documentary Watermark, “infrequent scenes of smoking”.

Ben Wheatley, the esteemed British director who has pushed the envelope in films such as Sightseers (“bloody violence”) and Kill List (“very strong bloody violence”), is all for information being available online, but finds the appearance on the black card “a pretty odd development”. “What are you supposed to do at that point as a viewer? It’s a bit late in the day.

“Why stop there? There’s lots of other things that offend or worry me. Maybe a warning for ‘generic plotting’ or ‘overuse of shallow focus’, or simply ‘not very well made’.”

On a more practical, shop-floor level, Kevin Marwick, manager of the independent Picture House in Uckfield, East Sussex, is more interested in being able to direct parents with small children to official guidance. “When asked the direct question, ‘Will a film will be too scary for little Tommy?’ it’s difficult for us to respond with any certainty because all children are different – this is when the BBFC advice becomes handy.”

But, tongue in cheek, he noted: “Many patrons can’t see the large sign pointing them to the toilet; I doubt they even notice the small print on the certificate.”