Former prime minister Sir John Major is to enter the General Election fray with a stark warning that a minority Labour government propped up by SNP votes would be a 'recipe for mayhem'.

With 16 days to polling on May 7, Sir John will say that a Labour-SNP administration would lead to 'weak and unstable' government and wreck Britain's economic recovery.

He will use a speech in the Midlands to paint a picture of an Ed Miliband government subjected to a 'daily dose of blackmail' by the nationalists who could bring him down whenever they wanted.

However Conservative former cabinet minister Lord Forsyth has warned the party's tactic of targeting a Labour-SNP link-up was 'short-term and dangerous' and could ultimately damage the Union.

In a BBC interview last night, the Labour leader insisted he would not be dictated to by the nationalists, even if he had to govern without an overall majority in the new parliament, saying: 'That ain't gonna happen.'

But Sir John will argue that, in practice, Mr Miliband would be forced to accede to the SNP's demands or face the collapse of his government.

'If Labour were to accept an offer of support from the SNP, it could put the country on course to a government held to ransom on a vote-by-vote basis,' he is expected to say.

'Labour would be in hock to a party that - slowly but surely - will push them ever further to the left. And who would pay the price for this? We all would. We would all pay for the SNP's ransom in our daily lives - through higher taxes, fewer jobs, and more and more debt.

'This is a recipe for mayhem. At the very moment our country needs a strong and stable government, we risk a weak and unstable one - pushed to the left by its allies, and open to a daily dose of political blackmail.'

Launching her party's manifesto in Edinburgh yesterday, SNP leader Nicola Sturgeon sought to allay the fears of English voters, insisting they would act 'responsibly and constructively' in the interests of the whole of the UK.

But Sir John will say that the SNP's 'driving ambition' is an independent Scotland and it would inevitably use its position to demand policies that favour Scotland at the expense - 'quite literally' - of the rest of the UK.

'That is no way to run a country. And nor is it remotely fair to England, Wales and Northern Ireland,' he will say.

The former premier will point to the way that Scottish Labour leader Jim Murphy has already suggested that funds raised from the party's planned 'mansion tax' in England could go to Scotland.

'If a Labour leader asks for that, how much more will the SNP demand?' Sir John is expected to say.

'And if this is the way Labour intends to behave towards England, how can they say 'no' to the SNP? And if Labour did say 'no', the SNP could withdraw support and bring down the government at any time.'

With the opinion polls pointing to a hung parliament with the SNP holding the balance of power as the third largest party, the threat of a nationalist link-up with Labour has emerged as the main Conservative line of attack in the election campaign.

The tactic has caused misgivings among some Tories. Lord Forsyth - who served as Scottish secretary in Sir John's government - warned that building up the SNP to undermine support for Labour in England could ultimately damage the Union.

'We've had the dilemma for Conservatives, which is they want to be the largest party at Westminster and therefore some see the fact that the nationalists are going to take seats in Scotland will be helpful,' he told the Guardian.

'But that is a short-term and dangerous view which threatens the integrity of our country.'

He said that David Cameron's call for 'English votes for English laws' in the aftermath of last year's Scottish independence referendum vote had 'shattered the unionist alliance against the break-up of the United Kingdom'.

Labour deputy leader Harriet Harman said Lord Forsyth's comments showed Mr Cameron was playing a 'desperate and cynical game'.

'Even Conservatives are now saying that he is prepared to risk breaking up the United Kingdom because the SNP represent his only hope of clinging to power,' she said.

Meanwhile former Labour minister and London mayoral contender David Lammy suggested that the party could 'do business' with the SNP after the election.

'I think we can win this election. But clearly, after the General Election, you would forge common alliance with parties that you can actually do business with and the SNP must be part of that story,' he told ITV News.

'I still think Labour can form the next government and that's what I am fighting for. But yes, there is common ground with other parties and the SNP would be included in that and we may need to enter into discussion after the General Election.'