Call me old fashioned, but I always thought Unionists were opposed to being anti-English.

Jim Murphy's latest foray into nationalist territory has been hailed in some surprising quarters. The Hammer of the Nats himself, columnist Alan Cochrane, was full of praise for the Scottish Labour leader's mansion tax grab on London and the South East.

"He's succeeded in proving he's a genuine class warrior", said the Daily Telegraph columnist, "who's not scared of taking on the English."

However, in the very same paper, fellow columnist and London Mayor, Boris Johnson, wasn't having any of it. "He's mugged London" he said, warning that Mr Murphy is playing with fire and fuelling demands for an end to the Barnett Formula.

So, it's Jim Murphy versus the Tory South. We'll take from the London rich to give to the Scottish NHS, he says. Next he'll be saying his favourite film is Braveheart.

Joking aside, there has been an extraordinary reversal of roles since the referendum. Mr Murphy was clearly serious when he said that he was going to stand first and last for Scotland. Where will he stand next? A Scotland-only referendum on Europe? Holyrood to have powers over corporation tax? A Scottish national anthem?

He's certainly not worried about antagonising London Labour any more. His bid to use revenue from the proposed tax on £2 million houses to pay for 1,000 more nurses in Scotland hasn't only left London Tories fuming. The left- wing Labour MP for Hackney, Dianne Abbott, accused him of an "unscrupulous attempt to buy Scottish votes with money expropriated from London". Fellow Labour prospective candidate for London Mayor, Dame Tessa Jowell, said he was trying to use London as a "cash cow".

Labour MP David Lammy, also seeking the Labour nomination, claimed Mr Murphy was trying to "siphon off" money from London which could be used to address what he calls the "desperate problems of overcrowding, poverty and homelessness" in his Tottenham constituency. We tend to think of London as a homeland for Russian oligarchs and dodgy bankers but it also has some of the worst social problems in the UK. Ms Abbott claims that child poverty in London is a third higher than in the rest of England.

"Ferrets in a sack" jeered the SNP's Stewart Hosie, about Labour's new internal "civil war". But it is just a little rich for the SNP to be getting so precious about all of this. After all, Alex Salmond has made a career out of picking fights with Mr Johnson and "London Labour" . They haven't exactly been shy about attacking the south-east of England in the past either for hoarding the wealth of the nation. For SNP supporters to be backing up the Daily Telegraph's fulminations against Mr Murphy's "tax on London" is darkly amusing.

Mind you, nationalists have in the recent past been careful to avoid picking fiscal fights with specific regions of England. This is because Scottish nationalists are only too aware that many in the north of England wouldn't be averse to a wealth tax being applied not just to London, but to Scotland. They say Scotland gets a bit too much of the cake, having the second highest GDP in the UK but also higher public spending per head than the North.

During the referendum campaign, Mr Salmond went to great lengths to assure his friends in the north that Scotland was really on their side. When he complained on BBC's Question Time about the disproportionate level of public infrastructure spending in London, he compared it with the rest of England rather than with Scotland.

In fact, the SNP has largely avoided picking fights directly with England - as opposed to the "Westminster establishment" - since the days of the abortive "It's Scotland's Oil" campaign in the 1970s. Scots didn't like to think of themselves as "grabbing" revenues that many believed should be used for the benefit of the country as a whole. Pooling and sharing. More fool them, you might say now that most of the oil has gone. But it remains the case that Scottish voters don't like to get involved in regional squabbles over resources.

The main problem with the Murphy Tax as it will inevitably be called is that it tries to earmark tax revenues raised in one area of England for a specific use in Scotland, for 1,000 more nurses. What Mr Murphy actually said was: "We will tax houses in London and the south east to pay for the 1,000 new nurses in the Scottish NHS. It's a real win-win for Scotland." That may only have been a rhetorical lapse into the crude anti-English nationalism that the SNP used to peddle. But that doesn't make it insignificant.

He is right, of course, to say that the mansion tax will mainly hit wealthy home owners in the south east. But to suggest that, in some way, he is able to take tax from one part of the UK to spend it in another is provocative and, as an astute politician, he must have known what it would provoke.

Mr Murphy is a former Scottish Secretary and knows anyway that this is not how the Barnett Formula, which calculates Scottish spending, is supposed to work. Scotland doesn't get a percentage of increased taxes raised in England; it gets a percentage of increased public spending in England, which is a crucial difference. Scotland is not subsidised by English taxes, as many Tory MPs claim, but gets a population-based share of increased spending by the UK Government.

Mr Murphy's supporters insist that this difference is academic. If a Labour government finances increases health spending, as Ed Miliband proposes by imposing a mansion tax, then surely consequential increases in Scottish NHS spending will be financed indirectly by the same mansion tax on London and the south east?

But it isn't just academic. The Barnett principle - which Mr Murphy claims to uphold - is designed to obscure regional transfers of wealth and prevent rows like this happening. At the very least it was a serious error in diplomacy.

"It is very regrettable that Labour should use divisive tactics," said Boris Johnson, "setting up one part of the country against another". Many of Mr Murphy's former colleagues in the Better Together campaign will have been nodding silently in agreement with that, while being puzzled at the way things have turned out since the Unionists supposedly won the referendum.

The Murphy plan seems to be to bamboozle the Scottish electorate by being "more nat than the Nats" and to lead them back into the Labour fold by staging confrontations with Labour MPs in other parts of the country. But this "blue on blue" is a risky strategy. Anti-Englishness can be habit forming. If two parties start competing on this same ground, it could validate the "wrong kind of nationalism".

We are used to saying that Scotland is different because its politics is dominated by two essentially social democratic parties, Labour and the SNP. It still is. But perhaps we now also have to say that they are two essentially nationalist parties. Labour is straying so far into SNP rhetorical territory that it may soon be difficult for voters to tell the difference.