The Prime Minister has warned of "the chaos and cacophony of two more referendums" in 2020 if the SNP "bundle" Jeremy Corbyn into 10 Downing Street.
Closing the Conservative conference in Manchester, Boris Johnson accused Nicola Sturgeon's party of having a "programme of total national discord".
He pointed to Labour's policy of giving the public a final say on Brexit if it is elected to government as well as the First Minister's plan for a second independence vote in the latter half of next year.
The PM also said he is putting forward "constructive and reasonable proposals" to Brussels to solve the Irish border question in his bid for a new Brexit deal.
In his keynote speech on Wednesday, Johnson vowed there would not be customs checks "at or near the border in Northern Ireland" in any circumstances.
He reiterated his promise that the UK will leave the EU on October 31 and "get Brexit done", with or without a deal.
The Prime Minister compared the Labour leader to 1980s-era Soviet leader Konstantin Chernenko, who was terminally ill when he assumed office.
Johnson said: "It now appears the SNP may try to bundle (Corbyn) toward the throne like some Konstantin Chernenko figure... reluctantly propelled to office in a Kremlin coup so they can get on with their programme of total national discord.
"Turning the whole of 2020 - which should be a great year for this country - into the chaos and cacophony of two more referendums.
"A second referendum on Scottish independence even though the people of Scotland were promised that the 2014 vote would be a once-in-a-generation decision, and a second referendum on the EU.
"Can you imagine? Another three years of this. But that is the Corbyn agenda."
He called the union "the successful political partnership in history" which he pledged to "protect and defend against those who would wantonly destroy it".
Johnson added: "And I say to Ruth Davidson as well, thank you Ruth, for everything you did."
The former Scottish Conservative leader quit her post shortly after Johnson entered Number 10, amid reported tension between the pair.
Interim Scottish Tory leader Jackson Carlaw this week reversed her opposition to a no-deal Brexit, saying the party would back that outcome over further delay.
But Johnson insisted to Tory delegates his aim is to strike a deal with Brussels in the coming weeks, saying his offer to the EU is a "compromise".
He said: "Today in Brussels we are tabling what I believe are constructive and reasonable proposals which provide a compromise for both sides.
"We will under no circumstances have checks at or near the border in Northern Ireland
"We will respect the peace process and the Good Friday agreement
"And by a process of renewable democratic consent by the executive and assembly of Northern Ireland we will go further and protect the existing regulatory arrangements for farmers and other businesses on both sides of the border.
"At the same time we will allow the UK - whole and entire - to withdraw from the EU, with control of our own trade policy from the start. "
Johnson added: "If we fail to get an agreement because of what is essentially a technical discussion of the exact nature of future customs checks, when that technology is improving the whole time, then let us be in no doubt that the alternative is no deal.
"That is not an outcome we want. It is not an outcome we seek at all.
"But... it is an outcome for which we are ready."
The Prime Minister maintained he wants to take the UK out of the European bloc on Halloween "come what may" - despite backbench legislation which requires him to seek a new Article 50 extension if he cannot get his Brexit plans through parliament.
Johnson brands the law the "surrender bill" and in his conference speech warned of "grave consequences" if Brexit is not delivered.
UK officials have made clear to the EU its Brexit proposals are a final offer and unless Brussels is prepared to engage there will be no more talks until after exit day.
The plan involves a "two borders for four years" measure that will leave Northern Ireland in a relationship with Europe until 2025, according to The Daily Telegraph.
It accepts the need for both a regulatory border between Britain and Northern Ireland in the Irish Sea for four years and customs checks between the North and the Irish Republic.
At Prime Minister's Questions earlier, Johnson's stand-in, foreign secretary Dominic Raab, confirmed MPs would be able to see the documents later on Wednesday.