THE SNP will enjoy a multi-million pound taxpayer windfall and cause a "profound" culture shock at Westminster if it succeeds in winning a landslide victory in Scotland at the General Election, according to senior Commons sources.

High-level talks have already taken place between Westminster authorities and the Nationalist leadership about the prospect of the SNP moving from the "margins to the mainstream" in the UK Parliament if it becomes the third party after May 7.

While the Nationalist leadership is forcefully stressing it is taking nothing for granted, opinion polls, since the independence referendum have consistently given the SNP a massive lead with some analysis suggesting it could even take as many as 53 of the 59 seats north of the Border.

With the Liberal Democrats having slumped in the polls, many pundits now believe the SNP will replace them as Westminster's third force behind Labour and the Conservatives.

One potentially significant change would be that, as an opposition party, the Nationalists would, with a much greater number of MPs, be entitled to a higher level of so-called Short Money.

These annual funds, provided by the Treasury, are given to opposition parties to enable them to run their Westminster operations, including hiring administrative staff and policy researchers.

The amount of money an opposition party receives depends on a formula that takes into account the number of MPs and votes a party receives at a general election.

In the current year, Labour, with the most seats - 256 - and votes - 8.6m - of any opposition party received the lion's share of Short Money as there is also an added component to fund the Opposition Leader's office. For 2014/15, Labour has received £6.7m.

In contrast, the SNP with six seats and 491,000 votes has this year received £187,000; £870,000 over the whole parliament.

But if the Nationalists were to increase their numbers significantly, then the amount of taxpayers' money it would get would shoot up considerably.

If, for example, the SNP won 30 seats and 600,000 votes, it could expect to receive around £600,000 a year or £3m over the five-year parliament. But if it won 50 seats and 1m votes - Labour in Scotland won 41 seats and 1m votes in 2010 - the Nationalists might expect to receive £1m a year or £5m over the parliament.

Such additional funding would allow the SNP to increase significantly its staff base at Westminster; at present it has just two press officers, one chief of staff, a business manager and a part-time researcher.

In 2010, the Liberal Democrats, when they entered the Coalition with the Tories, tried to keep its Short Money but failed because the party was no longer in opposition. This might be one consideration, among others, why the SNP would not want to go into coalition with Labour after May 7. By being in government, the Lib Dems has lost almost £9m in Short Money over the parliament.

Commons contingency planning is already said to have been drawn up in terms of where to accommodate a greatly increased Nationalist group but the talks with the Westminster authorities have gone much further, The Herald has been told, covering the prospect of SNP MPs being on every single Commons committee, chairing some of those committees, and being represented on key administrative bodies like the Commons Commission.

It has also been mooted that the SNP might wish to secure a place on the influential Intelligence and Security Committee.

While which committees a greatly increased group of Nationalists might seek to chair will be discussed through the "usual channels" following the election, it is almost certain the SNP would seek to oversee the Scottish Affairs Committee and possibly Work and Pensions; currently chaired by Scottish Labour MPs.

To underline how the SNP would be "at the heart" of the Westminster mainstream, if it became the third party, this would mean:

*its leader would have two questions at each weekly Prime Minister's Questions;

*SNP MPs would be called to respond to every ministerial statement and

*they would be involved in every debate with more spokesmen and women covering a wider range of policy areas.

Angus Robertson, leader of the SNP at Westminster, declined to comment on any private talks between his party's leadership and the Commons authorities but made clear if the Nationalists did well at the election, they would have a "profound impact" and the UK Parliament would be in for a culture shock.

"If the SNP is able to make the significant breakthrough that the polls are suggesting - and we take nothing for granted and are working for every single vote - it will come as a profound shock to many old-style Westminster politicians, who have always seen Scottish politics as a sideshow.

"Any hope that they had of Scotland putting up and shutting up after the referendum will be rudely awakened and the House of Commons will have to come to terms with the fact that Scotland and Scottish politics appears to be changing profoundly," he added.