Like a great murmuration of migratory birds, the millionaires of Britain are massing in the departure lounges. They are revving their private jets.

They are stubbing out their Cohibas in the smoking bars of Mayfair and preparing to leave our shores not just for winter but forever.

They may have decided they want somewhere warmer. They may want a city where the mayor actually fights back against street crime, and where the Rolexes are not snatched from their wrists.

But mainly they are going because they are simply appalled by Keir Starmer, and Rachel Reeves, and Labour’s intensifying war on the rich.

In today’s Mail Charlie Mullins, the heroic entrepreneur who founded Pimlico Plumbers and grew it into a £140million business employing thousands of people, explained why he could take it no more.

He is selling his penthouse. He is flogging both the Bentley and the Rolls-Royce and he is going to Dubai because he is damned if he is going to pay Labour’s grotesque and unnecessary taxes, and he is not alone.

People are appalled by Keir Starmer and Rachel Reeves’s intensifying war on the rich, writes BORIS JOHNSON

I am told that the hedge funds are already leaving Mayfair, with the tech titans hot on their heels. They are heading for Italy, where you can do a 200,000 euros deal for a flat rate tax. They are going to Madrid, to Portugal, to Switzerland, to the Gulf, and of course to the United States – anywhere to escape the jaws of the ravening Reeves.

Suddenly we are in the grip of a 1970s-style brain drain. At the very moment when Starmer has abandoned any attempt to control the entry of illegal migrants, wealth and talent is symmetrically starting to flee. This year alone it is estimated this country said goodbye to about 10,000 millionaires – because if Britain is about to be engulfed by Starmergeddon, they are saying to themselves, well, Starmergeddon out of here.

It is estimated that by 2028 we will have haemorrhaged 500,000 millionaires. Yes, you read that right: by the end of this parliament, half a million millionaires will have fled this country, like aristocrats fleeing the tumbrils of the French revolution. In the Labour government, and among the economically illiterate Lefties who now run this country, that prospect will be viewed with a flinty indifference.

So what, they will say, if we lose a few tycoons and entrepreneurs. Good riddance to these swankpots, say the Lefties: no one will miss them.

So what if the ostlers of Ascot and the ushers of Glyndebourne are suddenly missing their biggest-tipping customers. Does that really matter for the wider economy?

The answer is that it most certainly does. This exodus is a disaster for Britain, and for confidence in the British economy – which has already come under sustained rhetorical assault by Starmer and Reeves, the very people who should be shoring it up.

You don’t need to believe in the trickle-down theory, the idea that consumption by the affluent drives jobs and growth for millions of others – though that is very largely true. When I was Mayor of London, we had more billionaires than any other city on Earth. In fact, I used to boast that the city was their natural habitat, as the jungle of Sumatra is perfect for the orangutang, and their presence was not unconnected with the very high employment throughout the city.

Charlie Mullins is among many wealthy individuals choosing to take their money out of the UK, and over the next few months he plans to move every one of his assets

But even if you reject the trickle-down theory, you just need to look at the way the British economy works. You need the wealth creators for an even more fundamental reason – because it is frankly their ideas, their energy, their ambition that drives growth in the entire country.

Whether you are a plumbing king or a tech start-up, you are providing the innovation that keeps this country competitive and that attracts investment from around the world. Kill that culture, stifle those innovators, drive them away, and you will begin a downward spiral of decline.

You will have lower growth, and therefore lower tax yields; and then Starmer and Reeves will be forced to widen their nets ever further to pay for ever growing public expenditure, and the burden of taxation will fall ever more heavily on the entire population. It is already happening.

You can see it in the decision to take away the winter fuel allowance – not from the rich, but from 90 per cent of its current recipients. Why? Not because of some mythical black hole in the public finances, but because of the choices Labour have made.

They have opened our collective wallet to the unions, offering train drivers a 15 per cent increase in pay, even though they already earn about £60,000 per year. They have capitulated to the junior doctors, with no improvements in productivity (so much for the ‘reform’ demanded by Starmer).

They have bowed down to the public sector unions, and abandoned all attempts to cut the cost of government – which in its complexity and luxury is starting to resemble the later Roman empire. Take the so-called ‘Cabinet Office’, that mysterious entity at the heart of Whitehall, where people such as Sue Gray – former senior civil servant turned Starmer’s chief of staff – used to spend her time on the karaoke machine.

When Margaret Thatcher left No 10 in 1990, it numbered 600 people. When Tony Blair left office in 2007, it had reached 1,000. There are now about 8,000 of them. What are they all doing, and what is Starmer doing about it? Nothing, except of course giving everyone the right to work from home and not to be disturbed at the weekend by their boss.

They have junked our plan to cut government headcount by 66,000, and as the state gets bigger and bigger they are preparing to clobber businesses, to clobber individuals, to pay for it all; because the ghastly truth about Lefties – like Starmer and Reeves – is that they simply don’t care.

They actually believe in levelling down. They like lopping off the heads of the tall poppies. They want to wallop the rich. If you want to understand their mentality, think of that professor at Warwick University, who announced that we should all hesitate before reading to our children – because we would be placing less fortunate children at a disadvantage.

It sounds crazy, and it is crazy. It’s pure Pol Pot – but I promise you that IS genuinely how these people think.

When they see a hero like Charlie Mullins, they don’t rejoice in his success or his wealth or consider that in the 40-plus years he was at the helm, his firm paid £100million in tax. No, they wrinkle their nose. People are starting to wake up to what is happening, and what is coming down the track, and that is why they are leaving the UK.

The tragedy of it all, of course, is that the one group that will be ultimately protected from this madness is – as ever – the rich. They can either move, or shelter their assets, while absolutely everyone else gets plundered, in the ensuing downturn, by Rachel Reeves: your pensions, your property.

You name it, she’ll tax it. It is hard to think of an incoming government that has so completely misrepresented itself, or its intentions, to the public, in advance of an election.

Suddenly this country is going completely in the wrong direction. The caribou have looked up and realised that cold times are ahead, and they are off to pastures new.

And when the herd moves, it moves.