TONY Blair has lashed out at the SNP, branding Nationalism a “reactionary political philosophy,” which represented the politics of the caveman as he urged Labour not to lurch to the Left in wake of its crushing General Election defeat.

In a rare public intervention in domestic politics, the former Prime Minister issued a stark warning to his party not to repeat the mistakes of the 1980s, which had consigned it to 18 years in opposition.

With a poll placing leftwinger Jeremy Corbyn well ahead in the race to be the next Labour leader, Mr Blair described the London MP as the “Tory preference”, arguing the party could not regain power if it simply became a "platform for protest" against cuts.

He mockingly suggested anyone whose "heart says I should really be with that politics", should "get a transplant".

But Mr Corbyn, who used a speech to set out his economic plan, involving higher taxes on the rich and businesses, sharply rising investment in the economy and a crackdown on tax avoidance and evasion, dismissed the ex-premier’s broadside as “very silly”.

"Surely we should be talking about the situation facing Britain today, the situation facing many of the poorest people in this country today, and maybe think if our policies are relevant," he said.

Mr Blair also used his appearance at an event in the UK capital, hosted by Progress, the centre-left think-tank, to attack the SNP.

"We have to take the ideology of Nationalism head-on,” he declared. “Nationalism is not a new phenomenon. When they talk about it being new politics, it is the oldest politics in the world.

"It's the politics of the first caveman council, when the caveman came out from a council where there were difficult decisions and pointed with his club across the forest and said: 'They're the problem, over there; that's the problem.' It's blaming someone else. However you dress it up, it's a reactionary political force."

Mr Blair said the SNP had been allowed to "behave like an opposition" while in government at Holyrood.

"Look at literacy rates in Scotland; they've declined in recent years. There are problems with the Scottish health service.

"The SNP have a social democratic language but are simply managing an unchanged system. Labour is the only party capable of reuniting the UK. It has a big responsibility," he insisted.

But Mhairi Black for the Nationalists hit back, saying: "Tony Blair must still be smarting from Labour losing the Scottish Parliament election to the SNP in 2007 when he was Prime Minister and had dragged Labour so far to the right that it was barely recognisable.”

The MP for Paisley and Renfrewshire South claimed Mr Blair’s legacy “still haunts and damages Labour today”, and had led it into the sorry position of not even voting against the Tories' Welfare and Budget Bills this week, which had left her party as the real and effective opposition to David Cameron’s Government.

"On any reading of his record, Tony Blair was the one with the primitive policy; dragging the country into an illegal war in Iraq, at the cost of hundreds of thousands of lives and causing massive instability to the region, the ramifications of which we continue to live with.”

She said his criticism of the SNP Government suggested he was badly out of touch and presumably unaware that her party’s poll ratings this month had reached 60 per cent.

"While Labour continue with their infighting and recriminations, the SNP will get on with the job of delivering for Scotland and working for progressive politics across the UK," she added.

With Labour having lost all but one of its 40 Scottish seats in the SNP landslide, Mr Blair was asked if the party in Scotland might disappear “just like Woolworths”.

He replied: "No, I don't think we will. All political parties could in theory but I don't think we will."

He said he had spoken with some Scottish Labour figures who were "depressed, as you would expect" following the election drubbing.

But he noted that giving Scottish Labour advice felt like being someone who had spent the war at home telling veterans of the Somme how they could have done it better.

Mr Blair's intervention came as research by YouGov found Mr Corbyn was the first preference for 43 per cent of party supporters; way ahead of bookies' favourite Andy Burnham on 26 per cent.

Yvette Cooper, the Shadow Home Secretary, was on 20 per cent and Liz Kendall, the Shadow Health Minister, was one 11.

The study also forecast that if Ms Cooper and Ms Kendall were eliminated and second preferences redistributed under the alternative vote system, Mr Corbyn would beat Mr Burnham by 53 per cent to 47 per cent in the final round.

The suggestion that Mr Corbyn - originally seen as the rank outsider - could turn the the tables and win was greeted with dismay among leading figures on the centre and right of the party.

Tristram Hunt, the Shadow Education Secretary, warned Labour could be reduced to a "pressure group" that would not have "broad reach into all parts of the United Kingdom".

Meantime, John McTernan, a former special adviser to Mr Blair in Downing Street, turned on Labour MPs who had "lent" their nominations to Mr Corbyn to "broaden the debate", even though they did not want him as leader.

"They need their heads felt. They are morons," he said.