Oh dear, it’s kinky time again at The Times BFI London Film Festival. You know, when the threat of seeing on screen a bout of actual live sex, or even explicit simulated sex, is enough to drive serious film connoisseurs into their seats in paroxysms of giddy arousal.

In past London festivals Larry Clark’s Kids, David Cronenberg’s Crash and John Cameron Mitchell’s Shortbus did the trick. This year it falls to the estimable director and awards magnet Ang Lee, with his sensual spy thriller Lust, Caution, to provide the fireworks.

The director is renowned for his good taste and subtle emotionalism, displayed in films such as The Ice Storm and Sense and Sensibility, but it was with Brokeback Mountain that his talents reached new heights. With it, he made gay cowboy sex acceptable to mainstream America and cemented his status as a heavyweight auteur, the film winning him critical, commercial and Oscar success.

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So the news that he was tackling a compulsively erotic relationship during wartime China in Lust, Caution was greeted with intrigue, which was only heightened when American film censors gave it a prohibitive NC-17 rating for explicit sex.

Unfortunately, fans of arthouse smut who were hoping for a steamy hybrid of oriental bonkbusters such as The Realm of the Senses and The Lover will be sorely disappointed by a 2½hour film that saves its highly stylised eroticism for the dying minutes. The rest of us, though, will just be disappointed.

It opens in 1942, in Japanese-occupied Shanghai, and in the house of the stony-faced Mr Yee (Tony Leung). It will transpire that Mr Yee is a collaborator who executes resistance fighters, but for now we must circle endlessly around his mah jong table and listen to the inane gossip of his wife (Joan Chen), her buddies and the newcomer Mrs Mak (Tang Wei). A few smouldering stares soon reveal that Mrs Mak and Mr Yee have a history together and so we dutifully flash back to Hong Kong in 1938 to discover that Mrs Mak is Wang Jiazhi, a student activist who has inveigled her way into Mr Yee’s life.

Here, posing as the wife of a businessman, she hopes to get close enough to Mr Yee to allow her politicised classmates to kill him.

This Hong Kong section of the film is the lightest but also the least believable as Wang Jiazhi and her five student activists decamp to a hillside mansion and spend the entire summer working on this lone subterfuge (What, no jobs? No families? No cares?).

Worse still, they’re presented as a zany band of slapstick heroes in a broad tone that jars badly with the film’s mostly po-faced sincerity.

Needless to say, they fail to kill Mr Yee, who duly disappears to the mainland. And so on and on we go, back to Shanghai, this time to 1941, as the film once more reunites Wang Jiazhi with Mr Yee in an attempt, yet again, to get her back into his life, to seduce and assassinate him.

The problem here, of course, is that much like the film’s last-minute sex scenes (he likes it rough, she doesn’t seem to mind) everything in Lust, Caution, even the drama itself, is about delayed gratification. Which would be fine if the tension between Wang Jiazhi and Mr Yee were intriguing enough to sustain an entire movie but unfortunately it’s not, and what we’re left with is the dramatic equivalent of endless foreplay. Which, ultimately, doesn’t satisfy anyone.

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