The prospect of SNP participation in a future government of the UK was always going to rile a certain number of extremists south of the border.

But I was astonished when I read the latest article from the highly respected Scottish author, Allan Massie at the weekend.

Quoting Enoch Powell's infamous 1968 "rivers of blood" immigration speech, Mr Massie said that, if the SNP joined an electoral pact with Labour in Westminster after the General Election, there could be widespread political unrest in England.

I will quote him in full so that there is no confusion about what he actually wrote: "If the SNP has the effrontery to prop up a Westminster administration, English anger will have free reign. I don't say the rivers Thames and Mersey will literally foam with blood - but they might well do so metaphorically".

No one quotes from the most inflammatory political speech of modern times by accident. In doing so Mr Massie has brought a disgraceful taint of racial antagonism into a general election campaign. How could he?

And he goes on: "The mood in Scotland remains tense and ugly - and those living south of the Border should take note because the mood could quickly turn sour there too ... Democracy is at stake". I don't know what country (or planet) he is living on these days, but it certainly isn't Scotland.

The mood in this country is not "vile" or "ugly" but remarkably calm given the disappointment of the 1.6 million who voted for independence in September. The referendum was conducted with maturity and good humour, on the whole. The only significant unrest - the Murphy egg aside - was the Loyalist invasion of George Square, and even that was exaggerated.

Since September 19, there has been not a hint of violence on Scotland's streets. The legions of Yes voters have simply knuckled down to the task of furthering their aims through the ballot box. Yet this is now being represented as some kind of anti-English act of aggression.

Here is Max Hastings, historian and former editor of the Daily Telegraph: "We thus face the bleak prospect of five million Scots determining the fate of almost 60 million people in the rest of the UK."

The Scots have every right to participate in this election and SNP members have every right to be represented in Westminster and participate in government if that is how the dice falls. All MPs in Westminster are supposedly equal.

Indeed, was this not precisely the mission statement of the Better Together campaign in the independence referendum? That Scots would be better off participating in the UK political system than setting up on their own?

Allan Massie's Powellite diatribe is the extreme form of an attitude that has been widespread in the UK tabloid newspapers recently, though Scottish readers aren't often exposed to it because it is edited out of Scottish editions. The Sun, for example, has been running double page spreads about the threat from a "Frankenstein" SNP-Labour coalition.

There is a casual contempt for Scottish people who are routinely discussed in terms that would be inconceivable were they Afro-Caribbean or Jewish. Take this again from Mr Hastings: "Like the French and Greeks, the Scots seem immune to rational argument about their circumstances and prospects. They simply challenge the Westminster parties to declare who will pay most for their support."

Now, I don't mind a bit of political knockabout. The Tories' General Election poster depicting a smirking Alex Salmond with Ed Miliband in his top pocket is not, in my opinion, offensive. I could equally envisage a Labour poster of Ukip leaderNigel Farage with David Cameron in his top pocket.

And I don't really mind the relentless depiction of Scottish politicians prancing around in tartan with blue faces dragging England by the tail (though I did find the punchline on the Steve Bell cartoon in the Guardian about First Minister Nicola Sturgeon's fondness for "incest and country dancing" to be jarringly unfunny).

But "rivers of blood" is something else. The Scots are being depicted as a kind of "enemy within", an alien force seeking to exert control over England.

It is beyond the pale. We scribblers have a responsibility not to turn a general election into a witch hunt.