Illustration by Andrzej Krauze

David Cameron did not choose to put Scotland’s relationship with the rest of the UK at the top of the political agenda. That was done by Alex Salmond. But now the genie is out of the bottle the prime minister must seize the moment and refashion Britain’s constitutional landscape.

Cameron has an opportunity to score a triple crown of political victories. By offering to extend Scottish devolution he can be the Conservative leader who saves the union. By promising to balance Scottish devolution with a commitment to new arrangements for the government of England, he can radically improve his own party’s electoral prospects. And through these changes – with the introduction of city mayors and greater localism – he can be the PM who replaces one of Europe’s most centralised states with a political architecture fit for the 21st century.

To secure this triple crown Cameron needs to go further than his well-crafted speech of last Thursday, in which he lifted the veil on further powers for Holyrood but only if the Scottish people vote to stay in the UK. Alex Douglas-Home made a similar promise of jam tomorrow in 1979 but, once devolution had been rejected, there was no decentralisation of power during the subsequent 18 years of Tory government.

Cameron needs to set out his devolution agenda not after Scotland votes but before. Salmond has proposed “devo max”. A more palatable blueprint called “devo plus” has been drafted by the Reform Scotland thinktank. Under devo plus Scotland would not enjoy full control of tax policy but would be in charge of enough taxes to fund the whole Holyrood budget. Linking devo plus to a “no” vote would greatly increase the unionist camp’s chances of decisive victory that would settle the independence question for a generation.

Some say there is no need to offer the Scottish people any further devolution and point to opinion polls that suggest defeat for independence. But Cameron would be unwise to assume current opinion is fixed. Rupert Murdoch describes Salmond as the UK’s “most brilliant politician”. Scotland’s first minister may not appreciate the source of the compliment but the media mogul is not far wrong in his assessment. And it’s not just Salmond. In Angus Robertson MP the SNP has Britain’s wiliest political strategist. SNP coffers are overflowing. Scotland’s other parties are led by novices. And let’s not believe the independence referendum will be fought on the merits of the precise question posed. Other issues almost always pollute such campaigns. Last year’s referendum on electoral reform wasn’t, for instance, just about the merits of AV but also about Nick Clegg’s broken tuition fees promise. Salmond has deliberately timed the independence plebiscite for six months before the next general election. If opinion polls at the time suggest Cameron might be re-elected, you can bet that a central issue in the referendum campaign will be whether Scots want another spell of Tory rule from London.

Cameron must again face down the uber-unionists in his own party who opposed devolution from the very beginning and believe you stop independence by maintaining the status quo. In reality the UK will be kept together by ensuring that voters normally get the type of government they vote for. Current arrangements are unsustainable. You can’t have responsible government in Holyrood when, as now, MSPs control 60% of public expenditure in Scotland but only raise 6% of tax revenues. Devolution that ensures Scotland has to balance its budget is not another step towards independence but a final step towards a sustainable settlement. It will also give new hope to the Scottish Conservatives. One of the big reasons for voting Tory is to get lower taxes but so long as Holyrood is essentially a spending-only legislature that incentive does not exist.

But if Scotland is to get further devolution it is also time Cameron addressed the West Lothian question or, as Conservative MP Harriett Baldwin has correctly renamed it, the English question. The quid pro quo for introducing devo plus north of the border must be English votes for English laws south of the border. First proposed by William Hague a decade ago, the 2010 Tory manifesto promised to “introduce new rules so that legislation referring specifically to England, or to England and Wales, cannot be enacted without the consent of MPs representing constituencies of those countries”. Like too many other pledges this fell victim to coalition compromises but it is in the clear interests of the English voter, and the Tories, that the first steps are taken towards a federal UK. Within that new kingdom we can come together on issues of joint endeavour but be free to decide more policies at the level of England, Scotland, Wales and Northern Ireland.

Just as it’s sensible that Scotland doesn’t get English Toryism imposed upon it, it’s also right that England can have a majority Conservative government if it votes for it. The chronic failure of the Tories in Scotland means any Conservative leader needs a super majority in England to give the English the kind of tax and crime policies they want but Scottish voters, under devo-plus, could readily get for themselves.

Not addressing the English question, like resisting further devolution, endangers the union. If the 2014 referendum becomes a close-run affair one of the factors that could tip the Scottish people towards endorsing separation is a sense of hostility from the English. In the countdown to a vote the English sense of injustice might grow as contentious questions of the UK-wide distribution of public expenditure and of Scottish MPs voting on English-only services bubble up the agenda. Cameron can nip this in the bud by making an early commitment to some tangible form of English devolution. At the same time he can save the union, enhance his party’s electoral prospects and end outdated centralisation. Oh, and a fourth thing, he can wipe that grin off Salmond’s face.