In an emotional campaign speech in Perth, the Prime Minister urged people to "shout from the rooftops" over the remaining 76 days of the campaign if they wanted to stay in the UK.
He also accused the Nationalists of failing to answer basic questions about an independent Scotland, including its choice of currency, branding First Minister Alex Salmond "Alex the Unready".
Mr Cameron's rallying cry, at an event for his party's Conservative Friends of Union campaign, marked a shift in tone after previous interventions were dismissed by the SNP as "scaremongering".
Focusing less on the economic implications of independence and more on his personal pride in the UK's history and place in the world, he told supporters: "It's an issue of the heart.
"It would break my heart to see the UK break apart."
In a speech delivered without notes, he referred to his great, great Scottish uncle, Captain John Geddes, who fought in the First World War, and to Winston Churchill, who led Britain as a "beacon of freedom," in the Second World War.
Mr Cameron offered "five key things" in support of Scotland staying in the UK.
He cited achievements including the NHS, BBC and welfare state, and Britain's historic role ending slavery and opposing the Nazis in the Second World War.
He said Scotland was safer as part of the UK in a "dangerous, difficult and threatening world," highlighting today's ceremony to name the Royal Navy's new aircraft carrier, the Queen Elizabeth II as an example of Britain's military strength.
Mr Cameron said the UK, as a bigger market, was better placed to support business and encourage new industries, including Scotland's growing renewable energy sector, through good economic times and bad.
He also claimed Scotland could enjoy "the best of both worlds" as a result of further devolution to Holyrood.
He said the Scottish Parliament enjoyed "huge power" within the "solidarity and security" of the larger UK.
But he added: "I believe more should follow. This is vital. Voting No does not mean voting for no change."
Finally, to applause from the audience at Perth's Dewar's Centre, he said: "We should stop this argument that you can't be a patriotic and proud Scot if you want to stay in the UK."
He said Scots should not have to choose between pride in the Saltire and in the Union Flag, adding: "I think we need to shout from the rooftops."
In his only dig at Alex Salmond, he claimed the First Minister had failed to answer questions about an independent Scotland's currency, EU membership and the cost of setting up a new state.
"He's had years to think of the answers to these questions yet with 77 days to go, he's Alex the Unready."
The speech came hours after the UK Government announced a major investment in Glasgow's infrastructure.
Also on the campaign trail yesterday, Alistair Darling, the leader of the Better Together campaign, visited Midlothian-based engineering firm MacTaggart Scott, which issued a statement saying the "risks of separation far outweigh the benefits." The firm, which employs 350 people, is heavily dependent on Ministry of Defence contracts.
Meanwhile Finance Secretary John Swinney highlighted the Scottish Government's plans to turn away from the UK Government's austerity drive and maintain public spending in the event of a Yes vote.
He said an extra £1.2 billion would be provided in 2017/18 and £2.4bn the following year, funded through borrowing.
Speaking in Dundee, he said: "In the first year of an independent Scotland, our balance sheet is forecast to broadly match the UK's and public-sector debt will be falling as a share of GDP. Scotland will therefore start life with the opportunity to do things differently."