AS David Cameron partook in a bit of electoral cross-dressing by seeking to portray the Tories as the party of working people after Ed Miliband had donned the blue mantle of fiscal responsibility, a constitutional hand-grenade was quietly lobbed into the election campaign.

In among the Conservative manifesto plans for more free child-care, extending the right to buy and locking in a tax-free minimum wage, tucked away at the bottom of page 70 was a small paragraph about the proposal to extend the principle of English consent to financial matters such as how spending is distributed within England.

Now this could be argued this was a corollary of the Prime Minister's beloved English Votes for English Laws(Evel). But there was something more. English consent would, explained the policy document, extend to taxation as well, "including an English rate of Income Tax, when the equivalent decisions have been devolved to Scotland".

Within hours of the independence referendum result, Mr Cameron (in)famously stood on the steps of Downing Street and said, I paraphrase, enough about Scotland, what about England. Evel was born.

The Tory leader's comrades in arms in the referendum battle cried foul but Mr Cameron insisted the English voice now had to be heard.

But after an initial outcry, Downing Street sought to calm fears, insisting the vow to Scotland on more powers would be honoured and that addressing the English Question was not a pre-condition to this. The Smith Commission followed and agreement across all of Scotland's five parties was reached.

A key point of agreement was that income tax remained an issue to be decided by all UK MPs. Fears had been raised that any diminution of Scottish MPs' involvement in the main budgetary matters of the day would lead to them becoming second class citizens at Westminster. So the Commission proposals were published and the way forward settled. Or so we thought.

The implication of an English income tax rate is significant because 85 per cent of taxpayers are from England and once you split off England, then you begin to create a very different United Kingdom.

Professor Jim Gallagher, the constitutional expert, noted how the UK Budget had to add up and if you only had UK MPs deciding on how money would be spent but not on how it would be raised, then this would lead to great instability.

Of course, it could be that if English MPs alone did decide on how English taxes were raised, they would no doubt want to decide alone on how they were spent.

Jim Murphy, the Scottish Labour leader, denounced the PM's move as "belligerent", a betrayal of the Smith Commission agreement - echoing Defence Secretary Michael Fallon's personal attack on Ed Miliband by describing it as "a stab in the back" - and a craven attempt to woo Ukip-leaning Tories back to the fold.

The implication of the Conservative move is, indeed, great; that the UK could be on the way to being splintered.

Mr Cameron in his speech to launch the Tory manifesto stressed how above all else he was a patriot, declaring: "I love my country with all my heart." His critics might now suggest he was, of course, talking about England.