ENGLAND has nothing to fear but much to gain from a strong and responsible anti-austerity Scottish National Party holding the balance of power at Westminster, Angus Robertson, the Nationalists' campaign chief, has insisted.

As Tory high command continues to raise fears about the prospect of Nicola Sturgeon pulling the strings of a minority Labour government, Mr Robertson made clear that the "casting up of the SNP as some sort of threat" was more a sad reflection on David Cameron's Conservatives and Ed Miliband's Labour Party than on anyone else as, he stressed, polls continued to show the "most popular option" for Scots in the outcome of the General Election was Labour-SNP co-operation.

In an exclusive interview with The Herald, the Nationalist chief declared: "There's no reason for people in England to be fearful of the SNP doing well in the General Election. Of course, people in the rest of the UK understand that the SNP wants the best for Scotland.

"But the responsibilities we may be able to exercise at Westminster will be a benefit not just to Scotland but to people in the rest of the UK."

With the latest poll giving the SNP its highest rating of 49 per cent, raising the prospect of the Nationalists gaining around 50 of the 59 Scottish seats, contingency planning is already underway at Westminster in preparation for Mr Robertson's party becoming the "third force" in the House of Commons.

This, among other things, would mean more SNP influence, more responsibility and more resources with extra taxpayer funds through so-called Short Money rising from a current £870,000 with six MPs over a five-year parliament to around £5m with 50 MPs.

Mr Robertson explained that he was under no illusions the limits the SNP would have in any post-election arrangement with Labour but he noted: "If you hold the balance, you do hold a power and will be able to make an influence on key decisions. That will be determined by the electorate; they will decide how many MPs we send to Westminster.

"Obviously, the more MPs we have, the more we will be able to influence both the governance and also the workings of the House of Commons. With a greater presence, we will be able to hold the Government better to account as well, which has a benefit to Scotland and the rest of the UK.

"So it's a tremendous opportunity but I'm confident it will lead people to conclude that it's just the next stage in Scotland's journey towards home rule and independence."

Earlier this week following a bitter row over Trident, both the leaderships of the SNP and Labour effectively ruled out a formal "confidence and supply" deal.

Yet while Mr Robertson was insistent that "you cannot rule out the options for co-operation by progressive political parties at any stage", he stressed that "there's a growing understanding that the most likely outcome of the election and how arrangements would follow would be by a vote-by-vote basis".

The 45-year-old politician described himself as being "part of a very fortunate political generation in the SNP", noting how when he joined the party, it had just two MPs, one MEP, was polling less than 10 per cent and membership was relatively small.

Now, with more than 100,000 members, the SNP is the largest party in Scotland.

"The SNP is in the strongest position going into a Westminster campaign ever. We're competitive in all parts of the country, which is a first in a Westminster election. It's hugely encouraging but perhaps the most important ingredient is that we are taking nothing for granted. It's because of that and working extremely hard that we are doing so well," he added.