The Prime Minister will proclaim England to be part of "the world's greatest family of nations" while the First Minister insists that, following a Yes vote, the UK would remain Scotland's closest friend and partner.
With England's flag flying alongside the Union flag over Downing Street, Mr Cameron this morning beats a nationalist drum, declaring that for too long England's national day has been overlooked.
"Today more and more people are coming together on or around April 23, eager to celebrate everything it is to be English," he says in his message, making reference to a diverse array of English products from Newcastle Brown Ale and Cornish pasties to Downton Abbey and The Beatles.
But Mr Cameron says he wants people to "reflect on one of England's greatest achievements: its role in the world's greatest family of nations - the United Kingdom".
He adds: "In just five months, the people of Scotland will go to the polls and decide whether they want to remain a part of this global success story. So let's prove that we can be proud of our individual nations and be committed to our union of nations. Because no matter how great we are alone, we will always be greater together."
Some 300 or so miles north of London, Mr Salmond will embark on a charm offensive with northern England. He will deliver a St George's Day message in Carlisle Cathedral, telling local business leaders that the strong cross-border friendship will endure following a Yes vote on September 18.
He will say: "The ties that bind the nations of these islands will continue and flourish after Scotland becomes independent. You will remain Scotland's closest friends. Following independence, the social union etween the peoples of these islands will remain."
The First Minister will project what some have dubbed an "independence-lite" message, insisting, after a Yes vote, people would still go to work, visit family and friends and get married across the border. The monarchy would be shared, as would the pound, and trade would continue to flourish between the two neighbourly nations.
Mr Salmond, who has branded London a "dark star" that sucks in jobs and investment which otherwise would go elsewhere around the UK, will seek to curry favour in northern England, claiming an independent Scotland would serve as a "powerful economic counterweight to London, helping to rebalance growth across the British Isles".
Mr Salmond will propose the establishment of "special borderlands economic forums", should Scots vote Yes. "An independent Scotland will work in collaboration with our friends and colleagues in the north of England to improve economic circumstances and job opportunities for all our citizens," he will say.
Mr Salmond will add that the creation of the forums will be a "practical demonstration of co- operation and partnership between us; a partnership which will be strengthened by an outward-looking, prosperous, independent Scotland".
Former Labour Prime Minister Gordon Brown yesterday outlined what he described as the positives of Scotland remaining part of the UK, while business organisation the CBI was hit by further departures following its decision to register with the Electoral Commission as a No campaigner. The Law Society of Scotland, two more universities and government agencies all resigned their memberships.
Last night, John Stevenson, the Conservative MP for Carlisle, said the SNP leader's charm offensive would not change the fundamentals: that an independent Scotland would create
unnecessary red tape, in areas such as employment.
"We already have good economic links between Scot-land and England. Why try to alter something that is already successful and create rules and regulations where none exist at present?" asked the Aberdonian, who recently called for Scots to be banned from voting in the 2015 General Election if they opted for independence.
Meanwhile, Scottish Conserv-ative deputy leader Jackson Carlaw said of Mr Salmond's speech: "People in England will certainly recognise a dragon when they see it, particularly a toothless one. Meanwhile, the people of Scotland look forward to slaying the independence dragon on September 18."
Elsewhere, No 10 responded coolly to the narrowing gap between Yes and No to just three points in one weekend poll.
Asked if Whitehall would change tack, Mr Cameron's spokesman said: "The Prime Minister has said from the outset that it's important to make the case for the UK; there is never room for complacency."
The spokesman indicated that Alistair Darling was still the best man to lead the No campaign.