To make No a positive, first resolve the English Question

The former Chancellor is expected to say that 40,000 people cross the Border every year. 800,000 Scots live in England and 450,000 English-born souls live in Scotland. Which is all very well, though I don't ever recall the Yes campaign advocating ethnic cleansing. The single UK market, the former Chancellor will argue, promotes free trade, though again no-one is talking about breaking up the sterling zone, except perhaps George Osborne. "It is entirely possible to be a patriotic Scot and be entirely at ease with being British," Mr Darling will say. I don't think Alex Salmond would disagree – in fact he's used very similar words.

But enough platitudes. What Mr Darling needs to do today is state clearly and for the record what life after No will actually look like. What are those extra powers, if any, that Holyrood will acquire if Scots reject independence? Can Scotland remain in Europe if England votes to leave the EU? What will happen to Scottish MPs in Westminster, now that the UK Government is planning, according to press reports, to deprive them of votes on non-Scottish legislation? How will Scotland be financed after Barnett is no more? And what was that he was saying about the postal service being threatened by independence?

Events are moving fast. According to research published this week by Edinburgh University and the IPPR, your average English voter thinks that Scotland gets too much public spending, has too many MPs and is far too keen on Europe. But what's new, according to the Future of England Survey, is that they've decided to do something about it. Around 80% of English people want Scottish MPs barred from voting on English legislation and want Scotland to raise its spending from local taxation.

And they might very well get it. One London newspaper's front page yesterday was dedicated to a report that the Prime Minister, David Cameron, and his deputy, Nick Clegg, have agreed a radical solution to the West Lothian Question: English votes for English laws. The UK Government is about to finalise its response to the McKay Committee report, which earlier this year argued that it was no longer acceptable for Scottish MPs to determine the outcome of votes on domestic affairs in England when English MPs have no say over devolved matters in Holyrood.

The Coalition proposal is to ban Scotland's MPs from votes on measures that affect only England, such as crime, health, education and so on. This will be achieved by holding a special "fourth reading" debate after the normal three stages of passing a Bill through Westminster. In the final vote, non-English MPs will be excluded. If true, then it could herald a constitutional revolution, as profound in its way as Scottish independence.

For the first time there would be two categories of MP in Westminster – those with and without full voting rights.Scottish MPs would not be able to sit in the Cabinet because they lack the power to vote on key issues.Indeed, there would arguably have to be two Cabinets: one for English governance and another for UK issues like defence, foreign affairs and monetary policy. And here's the twist: these two "Cabinets" might have different Prime Ministers: a Conservative for English affairs and a Labour one for UK affairs. Labour is highly dependent on non-English votes to secure a majority in elections, while the Tories are increasingly becoming the "English party" with virtually no representation north of the Border.

These anomalies are why this scheme, originally proposed by the Liberal Prime Minister William Gladstone more than a century ago for Ireland, was never introduced. It amounts to a form of federalism without a federation, without a unifying federal level of government. The idea of turning the House of Lords into a Senate with geographical representation has not apparently been endorsed. Instead, the Westminster Parliament will become a de facto federal administration, which also happens to legislate for England.

Ed Miliband is understandably hostile, because it would reduce his hopes of forming another government. Labour still relies on those 67 non-English MPs to give it voting strength in Westminster, and if they were excluded from domestic English legislation, Prime Minister Miliband would be in office without being in power. Labour's plans to re-integrate the National Health Service could be blocked by the English MPs, assuming they had a majority in England. Labour would not be able to roll back Michael Gove's reforms to English education or reform the justice system.

This would be an extraordinary situation, and would make ordinary government impossible. Federal systems, like those in the United States and Canada, have two levels of government so that this collision of mandate does not arise. And English votes for English laws would have a profound impact in Scotland. The demand for Scotland to raise, in taxes, all or most of what it spends on services goes way beyond any of the proposals so far made by the Unionist parties. It is in effect, devolution max – the very option that is being denied to Scots in the forthcoming referendum on independence.

It was of course Scottish devolution that created the crisis of parliamentary legitimacy in England. It is unfair for Scottish MPs to vote on matters like university tuition fees in England when they have been abolished in Scotland. By what right should Scottish MPs vote on prescription charges in England? These are anomalies that need to be addressed – though we shouldn't forget that the reverse applied to Scotland for many decades, when Conservative governments in England imposed measures like the poll tax that had been decisively rejected by Scottish voters.

But these latest Conservative-Liberal proposals reek of political machination – a way of out-foxing Ed Miliband and securing the Lib-Con government in perpetuity. As with the EU, the Conservatives are under pressure from the march of Ukip, which advocates an English Parliament. Yet the risk of an ill-considered response to the rise of English nationalism could be more damaging to the integrity of the UK, ironically, than Alex Salmond's independence-lite, which speaks of continuity through the social union.

As with the uncertainty over Britain's future in Europe, the referendum campaign risks being overtaken by events. If Scottish MPs in Westminster are to lose voting rights, the Scottish electorate needs to know exactly how this would work. If Westminster is to become a de facto English Parliament, many Scots might reconsider their views on independence.

At the very least, Mr Darling needs to address this new English Question, and reassure Scots that they won't be pulled out of the EU and reduced to being a province of Londonshire.