When is the TV series Spooks not a TV series?

When it's a film, of course. This week, four years after the BBC1 spy drama revealed its last ridiculous plot twist, Peter Firth is back for a big-screen version. In doing so he is part of a now long and sometimes ignoble tradition, the TV spin-off.

Once upon a time TV was cinema's mortal enemy, seen as a Johnny-Come-Lately visual experience intent on stealing cinema's traditional audiences. The movies responded by going big - Biblical epics - and by going deep - introducing 3D. But when neither worked film studios realised that they would have to find an accommodation with the fledgling medium. And anyway, there were all these old films mouldering in the archives they could always sell on.

And so began a symbiotic relationship that continues to this day. Soon film-makers began to realise that audiences who grew up watching their favourite shows every week might venture out to the cinema if those shows were given the Hollywood treatment. The earliest example I can find (and yes I've just Googled it) is a 1954 film version of Jack Webb's cop show Dragnet.

By the 1970s the British film industry comprised of little else but sitcom adaptations. Everything from Dad's Army to Rising Damp was given a big-screen outing. Few of them benefitted from the experience, though On the Buses - made bizarrely by Hammer Films - was the most successful British film in 1971, outshining more traditional Hammer fare such as Countess Dracula and Blood From The Mummy's Tomb. Perhaps that shouldn't come as a surprise. Bob Grant's teeth were much more frightening than any vampire's.

By the eighties and nineties film producers who had grown up watching TV were keen to put their childhood heroes on the screen. And so TV series from the fifties and sixties - Star Trek, Dragnet (again), Mission Impossible - were all remade with varying degrees of success and budgets.

In the 21st century traffic has begun to flow the other way. In recent years we've had TV versions of Fargo and From Dusk Till Dawn and there are plans to turn Tom Hanks's comedy Big and Tom Cruise's sci-fi thriller Minority Report into TV gold (or dross).

The only question now is when will cinema begin to go further? Why just stick to comedy and drama?

I can see the trailer now. "It was a time of great confusion. A country was on the verge of losing the bread of life. The pies wouldn't rise. Every flan had a soggy bottom. Only one man and one woman could save the day. Mary Berry and Paul Hollywood star in The Great British Bake-Off: The Movie! This time it's perishable."

Spooks: The Greater Good is in cinemas from Friday.