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 in Scotland deciding if the country should become independent in a referendum 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''.
Yesterday a group of 130 company leaders - employing more than 50,000 people in Scotland between them - insisted that the case for leaving the UK ''has not been made''.
Mr Cameron travelled north to make the "b usiness 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 Scottish First Minister Alex Salmond after he threatened 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 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: "I f 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?"
The "s tability and predictability" of the Union are "precious commodities in an unstable, competitive world," the Prime Minister said.
"They're what investors, businesses and families tell me they want more than anything else."
He added that "r ight at the heart of that certainty is knowing what money you'll be using, and what it's worth".
But with three weeks until the referendum he said " the proponents of separation have no answer" to this question.
Mr Cameron said: " Yet again, the choice is clear: s tay together, and retain that certainty of a single currency for a United Kingdom - one of our greatest advantages. Or take a leap into the great unknown with the man without a plan for Scotland's currency."
The "solidarity" and "common purpose" of the UK helps fund public services such as pensions, Mr Cameron argued.
"All nations and regions in the UK lean on each other, in good times and bad," he said
"Take pensions. In the UK, the costs of maintaining Scottish pensions are shared across the UK. Under separation, Scottish taxpayers' spending on pensions would rise by £1.4 billion."
He went on to warn that if Scotland were independent there would be "n o Union to help out a bank in trouble, and that could put people's savings at risk".
Meanwhile in a "competitive, fast-moving world" he insisted that the "sheer scale of our union gives Scottish businesses a government that fights for them around the globe".
Scotland benefits from the "clout of the UK Government", he argued, as he highlighted the UK's position as a " top-six economy with one of the biggest diplomatic networks in the world".
The Prime Minister said: "O ur scale is a great, great advantage for Scottish businesses - helping you to walk taller and shout louder.
"But it is something we will only sustain if Scotland chooses these great advantages over the great unknown."
Mr Cameron said later that he was not "taking anything for granted" in the run-up to the vote.
"There's three weeks to go in this campaign, I'm absolutely passionate about the case for keeping the United Kingdom together," he added
"I hope that will be the result."
He also stressed that his commitment to delivering more powers for Holyrood in the event of a No vote.
"I think that process of delivering further devolution would help bring people together after this campaign," the Prime Minister said.
"I'm absolutely convinced that Nick Clegg, Ed Miliband and myself are utterly sincere in making this pledge."
But Tony Banks, the chair of the pro-independence group Business for Scotland, claimed the Conservative's speech was " one long and empty collection of trite, patronising phrases and empty rhetoric".
The businessman added: "On one of his rare visits to Scotland he failed, once again, to lay out any positive vision for the future of the UK. Maybe that's why he didn't want to take part in any of the debates about Scotland's future but chose, instead, to give a talk to a private dinner with no opposition present.
"Someone should have told him that this is not a time for soundbites but it looks like he felt the hand of history on his shoulder and shrugged it off so he could talk politics instead of vision.
"He gave the game away when he praised himself for giving the richest in the UK a tax cut but he didn't mention the massive increase in the use of food banks.
"He also claimed that one of the big benefits of the UK was stability and predictability - he should ask the oil and gas companies whether they were feeling that stability and predictability when they suffered smash-and-grab raids under both Labour and Tory governments."
Mr Banks said: " Scotland deserves better than anything that David Cameron, Ed Miliband or Nigel Farage are offering. We need government with vision and government that looks to Scotland's interests all the time. Day-trippers are great for tourism but they shouldn't be government ministers; a Scottish Government serving Scotland's interests is what we need and a Yes vote is the way to get that."