Tony Blair launched his most blistering attack yet on the Scottish Nationalists yesterday, saying that he “despised” them for attacking him over Iraq and cash-for-peerages when these were issues over which the devolved Parliament in Edinburgh has no control.

The Prime Minister, in an exclusive interview with The Times, also claimed that the idea that voters could back the SNP in the Scottish election on May 3 in the belief that it would not open the door to the break-up of Britain, was “nuts”.

Mr Blair was speaking at the end of a campaigning trip to Labour heartlands in the West of Scotland, after pleading with voters in an earlier speech not to give him “a kicking one last time on my way out of the door” but to concentrate instead on the implications of electing an SNP administration in Edinburgh.

“I will be out of the door but the consequences of an SNP Government will be there for the next four years and, if they [the SNP] have their way, for ever”, he told an audience in Glasgow.

Mr Blair used his visit to signal what appears to be a U-turn in Labour strategy as they struggle to fend off the threat of a resurgent SNP, still ahead in opinion polls only 19 days from election day.

Instead of placing major emphasis on ratcheting up fear among Scots over the economic and social consequences of independence, the Labour emphasis is now on the benefits and positive case for keeping the union with England. Instead of questioning whether Scotland can be independent, Mr Blair and his aides have decided to pose the question to Scots, “Why independence?”, in the hope that the large number of undecided voters will turn away from the SNP.

So far in the campaign, Labour’s dire warnings about independence have had little effect, mainly, most observers agree, because the Nationalists are offering a referendum on independence in 2010.

This has allowed the SNP not only to turn the election into a verdict on Mr Blair, but to tell voters that a vote for them is not a vote for separation and that they will get a chance to vote on a breakaway from the UK in three years’ time.

Mr Blair, however, attacked the basis of the SNP’s pitch to the electorate. He said: “The SNP say to Scotland, ‘Go on, there’s nothing to fear [about independence]. Why not?’ The answer is a better question, ‘Why? What is the reason for doing it?’ Of course Scotland can be independent, but at what cost and to what purpose?”

He told The Times that ten years ago the SNP made the case for independence on the grounds that the Scottish economy was relatively weak. However, now, Mr Blair said, it made the case for independence on the grounds that Scotland was an assertive, confident, spirited country with an economy that was doing well.

He said: “The idea we have got to challenge is that nationalism best embodies that spirit. In fact the idea of a strong, assertive Scotland within a strong UK is the best embodiment of that spirit.”

“Put it like this; if you don’t really believe in independence, then voting SNP is, at the very least, a crazy risk to take”, he said.

Mr Blair said that he had taken the time to listen to a recent speech by Alex Salmond, the SNP leader. “I couldn’t work out what he was saying was the reason for going independent”, the PM said.

“I do despise the way they [the SNP] will talk about Iraq or me or the so-called cash for peerages or issues to do with Trident when they know that none of these issues are the issues which are at stake at the Scottish election.

“I think there is something fundamentally not right in trying to conduct a political debate in which you actually want to drown out the difficult questions by putting up a great barrage on these other things.

“The SNP are trying to say to people — which is another trick in their campaign — that you can vote for the SNP and that won’t be a vote for independence. But the point is, it is. That idea is obviously nuts. If you vote for the SNP you are voting to start off down the track of independence with them [the Nationalists] in the driver’s cab.”

Mr Blair continued: “They may say that there is a chance to throw the switch at a later stage but they are going to be driving the train.”

Mr Blair is said to believe that the next week or so is vital to turning the Scottish election round. At the moment they are trailing the Nationalists because the SNP have successfully turned this election into a verdict on Mr Blair’s tenure as Prime Minister and have come up with policies popular on the door step — such as scrapping the council tax and replacing it with a local income tax.

Labour has tried to point out that the SNP comes with policies which, Labour claims, could cost every Scottish household an extra £5,000 a year and that the Nationalists’ plan for a local income tax is undermined by a financial funding gap.

However, the SNP campaign — fuelled by an unprecedented £1.5 million in donations — has always appeared one step ahead of Labour, presenting Alex Salmond, its leader, as a First Minister in waiting, who is consistently ahead of Labour’s Jack McConnell, the present First Minister, in polls.

However, while the SNP have been ahead for months, the party’s popularity is not matched by support for independence. Polls show that little more than one quarter of Scots support a breakaway from the UK, indicating that the Nationalists are successfully plugging into general disillusion with the Blair Government. However, the stakes for Mr Blair and Gordon Brown, his probable successor, could not be higher.

Mr Blair does not want his devolution legacy to be seen as unravelling into the break-up of the UK while the Prime Minister-in-waiting does not want to be taunted that he has power over England while Labour’s hated Nationalist opponents rule north of the border.

Tony Blair yesterday:

“People can give me a kicking one last time on my way out of the door. Except I will be out of the door but the consequences of an SNP government will be there for the next four years and, if they have their way, for ever . . . Of course Scotland can be independent, but at what cost? And to what purpose?”

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