David Cameron hailed the Union between Scotland and England as the "greatest merger in history" as he urged voters north of the border not to break away from the UK.
The Prime Minister said remaining part of the United Kingdom benefited both businesses and people north of the border.
With voters going to the polls in just three weeks' time, he said: "Let's choose openness over narrowness. Our great advantages over the great unknown. And let's stay together."
Mr Cameron was addressing business leaders at the CBI Scotland annual dinner in Glasgow on the day that some 200 company chiefs signed an open letter declaring that leaving the UK would be in Scotland's "best interests".
High-profile figures, including Stagecoach chairman Sir Brian Souter, Clyde Blowers boss Jim McColl and retired William Hill chief executive Ralph Topping are among those who have put their name to the letter, along with whisky boss Neill Clapperton of the Springbank Distillery and Michelin-starred chef Andrew Fairlie.
First Minister Alex Salmond described the move as "hugely significant" in the run-up to the September 18 ballot, adding it is "clear recognition of the massive opportunity that a Yes vote represents for Scotland's economy".
Earlier in the week a group of 130 company leaders insisted that the case for leaving the UK "has not been made".
Mr Cameron travelled north to make the "business case for Scotland in the UK", adding this was "something which matters to every man, woman and child in our country".
He said it was "economic opportunities that brought our nations together in 1707" when the Union was agreed, adding this was "the greatest merger in history".
More than 300 years on he argued that "our union and our prosperity are still closely bound".
He highlighted four "great advantages" from the Union - the opportunities it creates, its stability, its solidarity and its scale.
The Conservative leader also used his speech to hit out at First Minister Alex Salmond after he reiterated his stance that an independent Scotland would not take on its share of the UK debt if no deal could be agreed on a currency union with the rest of the UK - something already vetoed by the three main Westminster parties.
The SNP leader said on Monday that if the UK denied Scotland the "financial assets" of the Bank of England "then the UK will get stuck with all of the liabilities".
But Mr Cameron questioned this "new threat not to pay a fair share of debt", asking: "Has this really been thought through?
"You don't force someone to underwrite your future debts by threatening not to honour your past debts - that's not what any responsible government or person would choose to do."
Mr Cameron said the UK was "one of the oldest and most successful single markets in the world" as he highlighted the opportunities that come from having a domestic market of 60 million people and almost five million businesses.
"Scotland does twice as much trade with the rest of the UK than with the rest of the world put together," he said.
"Trade that helps to support one million Scottish jobs."
He highlighted links companies such as Scottish Power and Irn-Bru have with those south of the border, saying: "Scottish Power: it doesn't just generate power for Scotland - it generates electricity for Merseyside and Wales, too.
"Look at iconic Scottish drink Irn-Bru: it isn't just made in Lanarkshire - it is made down in Milton Keynes as well.
"That is what an interconnected economy looks like - an economy of opportunity."
He then warned: "If we pull that apart, if we make foreigners of our neighbours, business becomes tougher, trade becomes more costly and complex, and jobs become harder to find.
"So my case is this: our single market is one of our union's greatest advantages. If we stay together, Scottish businesses have better opportunities, Scottish consumers have more choice and Scottish people have more secure jobs.
"Why put that great advantage at risk by going into the great unknown?"
Mr Salmond, however, said Westminster politicians regard London and the South East of England as the UK's economic powerhouse, and as a result it is "inevitable that they will focus jobs and investment there".
The First Minister pressed Better Together leader Alistair Darling on what extra powers Scotland could get to boost job creation when the two men clashed in a TV debate earlier this week.
But the SNP leader said the former chancellor was "unable to name any job-creating powers guaranteed by a No vote" and added he was now "laying down the same challenge to the Prime Minister".
Mr Salmond said: "I have challenged David Cameron to a debate, now that Mr Darling has said he will refuse any more head-to-head encounters - but whether he accepts that challenge or not, the PM must use his visit to Scotland to say what powers Westminster is guaranteeing, to allow the job-creating businesses of Scotland to boost employment and the economy.
"If he fails to do so, the choice is clear - a Yes vote which guarantees the powers Scotland's companies and entrepreneurs need to create jobs and opportunities for future generations of Scots, or a No vote which will deliver absolutely nothing."
Tony Banks, chair of the pro-independence Business for Scotland group, said: "With independence we'll see proper support and promotion for Scottish exports and a business environment tailored to the needs of our businesses.
"We need a government that looks out for our interests rather just being obsessed with the bankers and financiers in the City of London, and we'll only get that with a Yes vote."
Earlier at the dinner in Glasgow, CBI president Sir Mike Rake hit out at the "uncertainty" being caused by the Conservative proposal to hold a referendum on Britain's membership of the European Union in 2017.
Mr Cameron dismissed fears that his strategy was "disadvantaging Britain in terms of investment".
But Sir Mike told guests that the proposed EU referendum - along with other political events such as the vote on Scottish independence and next year's general election - "contribute their own specific share of uncertainty".
He said: "We accept that calling a referendum on EU membership is a constitutional issue for government, but the ambiguity has already and is increasingly causing real concern for businesses regarding their future investment plans."