Scotland plans new independence vote to stay in EU if there's a Brexit

David Cameron's shock victory in the British general election makes an in-out referendum on the European Union within the next two years a certainty – and already anti-European MPs are manoeuvring to ensure the UK votes to leave.

What is a great deal more complicated is how Britain's potential exit from the EU will impact on the break-up of the United Kingdom.

The Scottish National Party (SNP), with its tally of MPs strengthened from six to 56 and now the third-largest party at Westminster, warns that a result in which English votes were weighted towards leaving the EU while the Scots voted 'in' would trigger another referendum on independence – this time much more likely, say pollsters, to result in a "Scoxit" from the UK.

"The SNP will be really crucial in this, because although they are pro-EU it would actually help them achieve their objective if we lost," says a leader of the umbrella campaign for British membership, which is set to be unveiled late this month.

Cameron, meanwhile, hopes to conduct a renegotiation of Britain's membership terms as fast as is diplomatically possible, beginning at next month's Brussels summit. He could then put this reformed arrangement to the voters as early as next year. In the post-election reshuffle of his ministers, key Right-wingers were included in the government to ensure loyalty and encourage the bulk of the party to follow Cameron's lead on the issue. But influential MPs have already begun to raise demands Britain may be unable to extract from other European leaders.

"It is entirely fair if people feel they should push this process further," said Graham Brady, chairman of the powerful 1922 Committee which represents Tory MPs, in calm but menacing tones.

He indicated that limited reforms to benefit entitlements, for example, would not satisfy many of his colleagues, and demanded freedom for Conservative politicians to campaign on both sides of the referendum.

David Davis, once Cameron's rival for the party leadership, is arguing that Britain should be allowed multiple opt-outs from European laws and regulations – a "pick and choose" attitude designed to irritate other EU nations and once dismissed by William Hague as tantamount to undermining the union.

At the 7 May election, 54% of voters chose MPs from Eurosceptic parties - Conservatives, Northern Ireland's Democratic Unionist Party (DUP), or the United Kingdom Independence Party (Ukip) which advocates withdrawal from the EU. Ukip won only a single Commons seat, but 13% support in the country. And Nigel Farage, remaining as leader despite failing for the third time to win a seat in parliament, has threatened to withhold support from anything other than the "right" sort of referendum.

Both sides are lining up to define the choice over Europe in terms that favour their desired outcome.

Campaigners for Britain's continued EU membership will demand a lower voting age and a referendum question fashioned for a positive answer.

After last September's independence vote in Scotland, the embryo 'in' campaign believes it can win if the electorate is asked to tick a 'yes' box – Scots wanting to stay in the UK were asked to vote 'no', which led to a nail-biting, last-minute majority for the union and a campaign where all the positive energy was generated by the secessionists.

Including younger voters is seen as vital because polls show they are more instinctively in favour of Britain's international links and value the right to live and work abroad in their own future education and career. Most of the opposition parties in Westminster are already in favour of votes at 16, with members of the Lords already agitating for broader voting rights in the referendum.

But the SNP has not committed itself to cross-party moves to secure a favourable question and expanding the franchise to 16- and 17-year-olds. With the pro-EU Liberal Democrats reduced to eight seats and Labour distracted by its leadership contest, SNP votes could be crucial. Cameron only has a majority of 12, making his premiership vulnerable to rebellions from the Right and the Left.

A recent ComRes poll injected some optimism into the pro-EU camp, showing a 56% to 34% lead for staying in, even if the referendum was held without reform. But the monthly trackers of public opinion tend to be narrower – and the Scottish experience shows that a campaign with the feel of a popular uprising against "those in charge" can quickly eat into a majority for the status quo.

The 'in' crowd are desperate to create a campaign that appeals to everyday Brits and their concerns, rather than parading business chieftains to argue about the benefits of export markets and streamlined regulation.

As Larry Summers, the American economist and Democrat politician, observed after the UK election: "A broad internationalist elite is not in fashion anywhere."

Analysts identify an era in which the public can be swayed by a mood of rebellion against those who hold the purse-strings rather than rational assessment of what's in their wallets.

"The politics of identity, be it Scottishness or Englishness, holds the key to voters' hearts," says Marcus Roberts, a Scot who works in London for Labour's Fabian Society.

During the election campaign, the Conservatives harnessed a nascent English nationalism by playing up fears of a Labour government pushed around by SNP insurgents. It worked but could now make for a wildly unstable future, with two interlinked crises at the heart of Cameron's second term as prime minister.

"It is no longer fanciful," wrote Philip Stephens, FT commentator, "to see Scotland leaving Britain and Britain leaving Europe."

David Cameron's political opponents gleefully predict the third possible split would be in the Conservative Party, torn between loyalists and "better-off-outers". But that would prove scant consolation.