The ignominy of it. All that cycling and recycling, the ghastly gloaming of low-wattage bulbs, dragging my old-lady shopping trolley up the hill, and it turns out that, far from accruing green indulgence, I'm going straight to eco-Hell. Our car, a grey Volvo estate, chosen because it was the least smug and look-at-me of family vehicles, a hearse designed by Ikea, is on Ken Livingstone's list of gas guzzlers that will have to pay £25 to drive into the congestion zone. My days of feeling morally superior to 4x4s - and never letting them out at junctions - must end. We're a Band G of brothers now.

Actually, since I almost never drive into Central London, and we're rid of the Volvo Pariah by April anyhow, Ken can do his worst. Besides, I am a stolid, 100 per cent backer of the congestion charge. It has gone far in breaking London's petrol habit: buses whizz (well, relatively) along, cyclists, once a Lycra-clad few, now flock over bridges, electric Noddy cars no longer endure mocking laughter. Which is why Ken's new rules infuriate me: while penalising me and the Chelsea tractors, he has also announced that from October thousands of small cars - Renault Clios, VW Polos and the like - can tootle around Central London absolutely free.

Just how does that make sense? A traffic jam of dinky Peugeot 107s is still a traffic jam. And if I was a wannabe WAG wedded to my monstrous, bull-barred, child-killing off-roader, I'd save it for the weekend and buy myself a nippy girl-car for the week. How can making it economical for rich people to buy second cars be green? And while a Fiat Punto emits around 100g of CO2 per km less than a Jeep Cherokee, that is still 100g more than a bus passenger or a cyclist.

When did the congestion charge turn into the emissions charge? Of course everyone wants to combat global warming, but the initial point was to have a city that moved and functioned. Getting people out of their cars has a far greater impact on CO2 than just decanting them into smaller ones. How many cyclists will now retreat into vehicles, making the roads more perilous for others? Besides, a tax on emissions alone is crass. The wily old car manufacturers are catching on fast: a Land Rover Freelander 2.0 diesel emits only 205g and will only pay the £8 charge. What is the virtue in streets blockaded by quasi-green tanks?

“I hate cars,” Ken said years ago. “If I ever get any powers again I'd ban the lot.” And there was purity in his intent until this week. When I ring up Transport for London to clarify why small is suddenly beautiful, the press office struggles to explain.

“We identified that some people still need to drive around Central London.” But who and why? “Well, the disabled...” But they're exempt from the charge anyway. Who else? “Er, we'll get back to you.” They never do.

What TfL cannot admit, of course, is that Ken has an election to win. His announcement echoes George Osborne's fortune-transforming proposal at the Tory party conference to tax non-doms while raising inheritance tax thresholds. And 4x4s are the engineering embodiment of non-doms with their moneyed, arrogant disregard for the city and wider humanity. Yeah, stiff them - and the Ferraris, Rollers and Bentleys - then reward the ordinary guy. But does it figure that the people's vehicle, the planet's saviour, is the Ford Fiesta? I thought it was the bicycle and the bus. Anyhow, lots of voters own ickle runabouts, or very soon will, and with polls saying that Boris is only 1 per cent behind, maybe that will be enough four-wheeled bribes for Ken to win. But watch out, suckers, because TfL says if a sudden influx of My Little Polluters gridlocks the streets, Ken will reverse his decision and all the Band B cars will have to pay the charge again.

This grotesque cynicism leaves me in deep electoral despair. Over the past four years, like many others, I've had the creeping realisation that the Ken I backed delightedly in his first and second terms now sees London only as a monument to his ego, his sole goal perpetuation of his own power. Let him swig single malt for breakfast, let others deal with the cronyism and dodgy dealing that infest every long administration. What saddens me is how a once Tube-riding, genuinely democratic Mayor now seems to view London as a huge architect's model, a grand design, peopled by tiny, silent, plastic figurines.

Consorting these days with developers and global capitalists, he sees himself as the architect of a masterplan: ordinary Londoners could never compute such complexity. He is the macro thinker, bored and disdainful of our piffling micro lives. We may think that encouraging London's population to grow from 7.4 million to 8.5 million will not leave much room to breathe. But he scorns those who say the city is already a crush, a daily elbows-out battle for resources, with every inch of green space being in-filled, and school rolls, dental and doctor registers bursting.

“I'm not worrying about what happens when we get to nine million,” he says breezily. “I won't be Mayor then.” And the white working class in outer boroughs like Barking, who see their neighbourhoods transformed into ethnic ghettos in a matter of years, are BNP stooges if they feel querulous or usurped.

Or enemies of progress, deniers of the future, like those of us who love London's human scale, its heritage and architecture, feel furious when the Mayor boasts that a new glass and metal hard-on - a Shard or Cheesegrater, Helterskelter or Walkie-Talkie - will spring up along the Thames, forever destroying ancient vistas, every 18 months for the next 15 years. When did we sign up to that vision of London's future? Or give Livingstone the power to overrule any planning opposition? Somehow we find ourselves with a Labour Mayor who wishes our capital was cut loose from Britain, so it could feed, without tiresome regulation, the gaping maw of international finance; as Ken puts it “a Singapore of the West”.

So who to vote for in May? The unlikeliest leftie friends are backing Boris for his bounce and bicycle, because it's a vote for change and mischief that will add to the joy of the nation. Who can see him, come the next London bomb, voicing the city's grief? And since when have Conservatives ever actually conserved anything?

But then I'd never have believed Ken's brave congestion gamble would end in a gridlock of shiny little cars.

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