A Labour-SNP alliance after the General Election would spark a constitutional crisis and cause "absolute fury" in England because separatist votes would be used to prop up a left wing Miliband government.

The Conservative outcry comes as William Hague, the Commons Leader, will this morning set out the party's proposal on English Votes for English Laws(Evel), regarded by it as a fair constitutional redress to England for the extra powers being offered Scotland.

But Tory backbenchers have warned the party leadership not to water down the proposals to ban Scottish MPs from so-called England only laws.

Prominent Conservative Bernard Jenkin branded any diluted option "a disastrous sell-out," saying: "It would continue the denial of English democracy. The English would just see Scottish MPs interfering in things that have nothing to do with their voters."

The two main options being considered by the Tory leadership are whether to ban Scottish MPs from all parts of an England only Bill or allowing them to participate at certain stages, most crucially the final one.

But Conservatives have also expressed alarm at the prospect of a Labour government propped up by SNP votes; what some Tories have dubbed "the nightmare scenario". Already, Nicola Sturgeon, the First Minister, has made it clear Nationalist MPs would vote on England's health service because any changes would have a budgetary effect on Scotland's NHS.

One senior Conservative said: "Such a scenario would undermine the Union. These are not Unionists, they're separatists whose stated agenda is to manipulate things to get whatever they want. If Labour go down this road, they will pay the price by losing more seats in Scotland. They will become an English party."

A party colleague said a Miliband administration kept alive by SNP votes would enrage English voters. "There would be absolute fury. It would be hypocrisy in the extreme from the SNP, who until now have acted honourably in not voting on matters which have not affected Scotland. If they started voting on English only matters, it would be utterly divisive."

Tomorrow, Gordon Brown, the former Labour Prime Minister will lead a Commons debate on Scottish representation at Westminster when he will argue against Evel. He is expected to say that banning Scottish MPs from Commons votes, most notably on the Budget, would create two classes of MP and undermine the very United Kingdom people fought so hard to save in last year's referendum campaign.

Meantime, Lord Wallace, the Advocate General for Scotland and the former Deputy First Minister, highlighted how difficult it could be to work out what precisely was an England only Bill, highlighting the most cited example - the tuition fees Bill of 2004 - as a myth.

"There is little doubt that the decision of the UK Parliament to provide for top-up fees at English universities had a very real effect on Scotland; even to the extent of legislation having to be brought in(at Holyrood) to address potential consequences."

He added: "Determining whether a Bill might have an effect on Scotland may not always be straightforward."

Elsewhere, MPs were on course to pass the parliamentary motion to devolve to Holyrood the power to enable 16 and 17-year-olds to vote in Scottish parliamentary elections, beginning with the poll in 2016.

Alistair Carmichael, the Scottish Secretary, said, that while at present there was not a consensus on extending the franchise to general elections, there was "every reason to believe that the tide has turned in favour of that outcome".

He added: "When it comes to extending the franchise in this country, the liberal, progressive argument always wins in the end and afterwards there is consensus that it was the right thing to do."

David Cameron is currently the only main party leader in Britain opposed to giving 16 and 17-year-olds the vote. He said he understood the case being made to extend the franchise but "18 is a better age; it's the age when so many other things kick in and it would be better to save that voting age to 18".

No 10 reiterated how the Prime Minister would be happy to have a Commons debate and, indeed, a vote on whether to give the UK's 1m 16 and 17-year-olds the vote. But this is not likely to happen before the May 7 General Election.