Independence poses a threat to both levels of safety and security and the future of defence jobs in Scotland, Defence Secretary Philip Hammond has told the Conservative Party conference.
The Scottish Government's defence plans under independence are not credible, while the SNP administration is "deluded" over maintaining current levels of shipbuilding, Mr Hammond said.
Addressing the party's Scottish conference in Edinburgh, with just over six months to go until the referendum, he argued that the scale of the UK Armed Forces and the size of the defence budget means the UK is "ready to meet the broad range of security threats we face", including cyber attacks.
"A separate Scotland could not hope to develop the same level of protection and resilience," Mr Hammond said.
He told delegates that the defence industry in Scotland employs around 12,600 people, generating sales in excess of £1.8 billion a year.
"The construction of the Royal Navy's new aircraft carriers alone - taking place just across the Forth Bridge, has already delivered hundreds of millions of pounds worth of work for the Clyde and Rosyth, with thousands of jobs in the shipyards linked to the programme," he said.
"But the truth is this: the overall drumbeat of shipbuilding work for the Royal Navy - from the Type 45 Destroyers, to the Carriers, to the Type 26 Frigates - is just enough to sustain one complex warship-building yard. For the whole UK.
"So when the separatists talk about maintaining warship building in Scotland to meet the needs of a separate Scotland, they are either deluded or they are seeking to delude.
"Whether it's the range of military capabilities; the strength of our geopolitical influence; or the number of domestic defence jobs we are able to sustain, it is absolutely clear that replicating the UK's level of safety and security, let alone sustaining the levels of defence employment under a separate Scotland is probably impossible, and would certainly far exceed the capacity of the budget the SNP say they would allocate to defence."
Mr Hammond also used his speech to attack the SNP over its response to news of a radiation leak at the Vulcan test reactor at Dounreay.
"Whether the separatists could exercise the self-discipline that matters of defence and security require is another question.
"And one that has had, in my opinion, quite a lot of light thrown on it by the blatant attempt over the last week or so, to make political capital out of the need for secrecy around aspects of our submarine programme - including the test reactor at Dounreay."
SNP defence spokesman Angus Robertson said: "Philip Hammond should have come to Scotland and apologised for the UK Government's secrecy over radiation leaks at HMS Vulcan in Dounreay.
"It speaks volumes about Philip Hammond's cavalier attitude to the safety of people in Scotland that he puts unacceptable levels of secrecy above environmental safety.
"That is a completely unacceptable attitude and shows that he clearly does not appreciate just how serious an issue the MoD's cover up is."
He added: "We will take no lessons from Philip Hammond when it comes to defence after his Government callously handed redundancy notices to thousands of serving forces personnel.
"Under Westminster Scotland has nuclear weapons that we don't need but not the naval vessels and marine defences that a nation with our size of coastline and strategic position should have.
"A yes vote this year will ensure that decisions on Scotland's defence are made in Scotland and always reflect the needs of people in Scotland."