A top-sliced licence fee will trigger the BBC's destruction

What should a government in trouble do? First, expose no new flanks to public anger. This is no time to start an epic battle over the fate of the BBC, which is more loved and trusted than any government can hope to be. Every poll shows the BBC is a national treasure - yes, even when it blunders and is never quite as good as its high rhetoric. Yet for reasons that are essentially frivolous, Labour is toying with its demise.

Can James Purnell, the culture secretary, really be serious when he talks, as he does, of top-slicing the BBC licence fee to spread the money among other broadcasters? Talking to him at last weekend's Fabian conference, he confirmed he was indeed.

He has, mercifully, ruled out an Arts Council of the Air commissioning programmes from all and sundry across all outlets. The bureaucratic and artistic nightmare of choosing individual worthy programmes and separating them from commercial programmes bore very little close inspection. But yes, he says, the BBC needs competition from other public service broadcasters. Indeed it does, but not by first cutting the BBC back. Now, top-slicing the licence fee to subsidise others is suddenly becoming the government's way to fund an ailing Channel 4. This very bad idea is in danger of gaining ground unless a sufficient public outcry stops it dead in its tracks. The cabinet must not be bamboozled by techno-speak: throttle this now.

Here's why: the BBC reaches well over 90% of the people with its many services. Three independent recent polls show the licence fee is not only acceptable, but a majority of people would pay more than the current level to keep the BBC. Transparent taxes for specific services are increasingly popular: you see what you get for your cash. If that money were to be disbursed widely to all kinds of providers - perhaps not just Channel 4's outstanding news but to ITV's South Bank Show, to Sky News, the History Channel or who knows, even to Guardian podcasting - that hypothecated money loses its recognisable link with the services paid for and risks becoming unacceptable.

The BBC has many dangerous and powerful enemies. Rupert Murdoch has his press lobbing relentlessly against the BBC, wanting it reduced to a US-style public subscription service offering only education and information, and no competition with the commercial sector. ITV is ailing and tries to blame the BBC, though analysts say its own disastrous management and OnDigital failure caused its present plight. Other predators eye the licence fee. If the BBC does well, enemies claim it's unfair competition, but if its ratings sank badly, they'd say it no longer deserved a universal licence fee.

So the idea of top-slicing the licence fee has dangerously influential friends whispering in the government's ear: the Tory benches are eager to give them hope. Taking a smallish chunk of BBC cash out for Channel 4 might seem a relatively harmless option, but once Labour breaches the link between the BBC and the licence, it opens the floodgates to a future Tory government to go much further. Thin-end-of-the-wedge arguments are often weak - but this lets out a genie that will devastate the BBC.

The danger is that clever, dynamic ministers like Purnell itch to do something emphatic and new. To rubber stamp what already works goes against the grain. The BBC is always a problematic outfit, an odd beast, rightly at odds with both government and opposition - too big, too brash, too left, too right, too staid, too daring, too popular, or not popular enough. But there it is, in all its contradictory magnificence. Will Gordon Brown really want to be the man who caused the destruction of Britain's most powerful global brand, the one authentic emblem of Britishness?

There are good reasons why Purnell and the media advisers round both him and the prime minister look at the convergence of internet, mobiles and TV and puzzle over the future. But that's no reason for all the conclusions that subsidising the national broadcaster is an anachronism. Don't listen to the technical arguments: it doesn't matter how the BBC is accessed, so long as it can be accessed on every arising technology. What matters is the quality of the content, which often comes last in new media futurology debates.

Of course the quality is variable and subjective. Personally, I often want to throw things at the BBC's "flagship" News at Ten, with its empty gimmicks, sending presenters pointlessly to link programmes from the McCanns' apartment or Beijing, emoting over crimes without context, failing to report the rest of Europe with nightly sins of commission and omission. But all the same, in a hurry for reliable information, where does the whole world go? To the BBC's brilliant website. We could mull the relative merits of Cranford versus Lark Rise. We could despair at £18m for Jonathan Ross, or pause for a moment's gratitude for all of BBC radio. Add here your own delights and disgusts.

But imagine the idea of public service broadcasting increasingly dispersed among myriad commercial outlets, pepper-potted in among ads and hard to identify, while BBC channels were drained of funds yet again. Gordon Brown already cut the licence fee, while making the corporation pay for national digital switchover. The BBC does need competition: Channel 4 news beats it every night. But if Channel 4 wants public subsidy, it needs less reality TV and a return to its original wildchild remit: brave, experimental and haphazardly bad and brilliant. But don't cut the BBC to do it. Why not help fund it with cash released from selling off the analogue spectrum after the digital switchover (courtesy of the BBC)? There could be windfalls on booming communications industries. After all, what's the use of new devices that do everything at once if there's still nothing good on?

Next winter Purnell responds to two reviews on the broadcasting/communications future. There are big questions that can never be answered: how do you measure what good the BBC does to national life? But the cabinet only needs to ask itself this: do you really want to pick a fight over the BBC? If so, they should remember what strong passions are rightly aroused. Talking to director general Mark Thompson yesterday, he will fight this to the end. Labour has paved the way for a Tory government to privatise much of the NHS, education and job centres. Does it really want to breach the founding principles of the BBC too?

[e-mail]