Conservative leader David Cameron has strongly defended the Union, pledging to fight the "ugly stain of separatism seeping through the Union flag".
Mr Cameron delivered his speech in the shadow of Holyrood
Speaking in Edinburgh, Mr Cameron said he believed the Union between Scotland and England was more fragile than ever.
He said there were those in England who wanted to see the Scottish Government succeed in its independence goal.
The SNP accused Mr Cameron of delivering a "negative message about Scotland in negative language".
The 300-year-old Union between the two nations was the only topic in a speech delivered by Mr Cameron as he travelled to Scotland to meet Scottish Tory leader Annabel Goldie.
He pointed out that just two days after the 300th anniversary of the Act of Union, Scots had voted for an SNP government at Holyrood.
In a bid to combat the SNP, the Tories, Labour and Lib Dems joined forces last week at the Scottish Parliament to vote through plans for a constitutional commission to look into the future of devolution.
Speaking in the shadow of both the Scottish Parliament and Holyrood Palace, Mr Cameron said: "It is my desire and my duty to help shape the future, and the future of our Union is looking more fragile, more threatened than at any time in recent history."
He added: "The SNP now promise to deliver independence in 10 years and at the same time there are those in England who want the SNP to succeed, who would like to see the Union fail.
"They seek to use grievances to foster a narrow English nationalism. I have a message for them: I will never let you succeed."
Prime Minister Gordon Brown, then chancellor, repeatedly said that Scotland and England were "stronger together, weaker apart" prior to the Holyrood election.
The Tory leader echoed that sentiment by arguing that the two countries were stronger, safer, richer and fairer together.
But while his speech echoed Mr Brown's words, Mr Cameron was critical of the Prime Minister's proposals on the subject of Britishness.
"He approaches the whole question of national identity like a sort of brand manager trying to launch a new product on the market or some kind of spin doctor trying to revive the reputation of a failing government department," Mr Cameron said.
"That's the thing about frustration. It can be easy to blame your neighbours, but what we should be doing is blaming Labour."
He said he believed that was why Scots had backed the SNP in May's Scottish Parliament elections.
Ms Sturgeon is confident of winning any independence referendum
Responding to the speech, SNP Depute Leader Nicola Sturgeon MSP pointed out that Mr Cameron had stated before the Holyrood poll that Scotland could be a successful independent country.
She added: "The Tories are facing both ways on Scotland. Just last week, the Tories north of the Border signed up to a commission to boost the financial powers of the parliament which would make us wealthier.
"Whereas the Tory drive at Westminster is about appealing to a south of England agenda and cutting Scottish spending.
"Instead of coming to sell a negative message about Scotland in negative language, David Cameron should support the right of the Scottish people to choose their future in a democratic referendum."
Labour's Jack Straw said: "Mr Cameron's plan for English votes for English laws is dangerous and unworkable."
Liberal Democrat MP Alistair Carmichael added: "
"Such constitutional illiteracy plays into nationalistic hands on both sides of the border."