Scottish regiments were believed to be in the firing line, with the military said to be making a series of cuts to balance the books.
The Prime Minister today insisted the Black Watch was safe as he unveiled the biggest defence investment in decades.
Responding to a question from the SNP’s Westminster Leader Ian Blackford, the PM shouted “yes, yes” as he was challenged over the regiment’s future.
He said: "I can certainly give the right honourable gentleman that guarantee. He seems to be a veritable geyser of confected indignation.
“Of course we're going to guarantee the Black Watch.
“It's preposterous to listen to the Scottish National Party talking about their desire to support spending when everyone knows fine well that it's thanks to UK-wide investment that we're able to deliver not just the Black Watch, and DfID [Foreign, Commonwealth & Development Office] in East Kilbride, but a fantastic programme.
“Under his plans, it's not just that there will be no deterrent, there will be no ship building, and no Black Watch in the land of the SNP.”
The announcement was welcomed by Scottish Tory leader Douglas Ross.
He said: “I called for reassurances that the future of the Black Watch would be safe and secure for years to come and I’m delighted that we now have that confirmation.
“The UK Government’s £16.5 billion boost to the armed forces will deliver real benefits to Scottish shipbuilding and the thousands of service personnel based across Scotland.
“The Black Watch plays a vital role in our armed forces and is a real symbol of our shared history, so it’s very welcome that its future is secured beyond doubt.”
Mr Blackford had earlier demanded support for the Black Watch and continued speaking while the PM shouted “yes” from the big screen.
He said: “In the SNP, we support a refocusing on the contemporary threats that we face.
"We need to readjust our defence capabilities for the modern world. It is especially important that a focus is given to issues such as cyber security.
“But what we do not accept is the priorities of this Government and the threat of the disbanding of historic regiments such as the Black Watch.
“Disbanding the Black Watch would show that the promises made to Scotland during the Scottish independence campaign have been broken, buried and forgotten by this government.”
It came in a speech that saw the PM set out a £16.5bn increase over four years to the defence budget that will ensure the UK remains the second highest defence spender in Nato.
Responding to the announcement, Labour leader Sir Keir Starmer accused the Government of lacking a “clear strategy”.
He told the Commons: “So we welcome this additional funding for our defence and security forces and we agree that it is vital to end what the Prime Minister calls – I have to say, with complete lack of self-awareness – an era of retreat.
“But this is a spending announcement without a strategy.
"The Government has yet again pushed back vital parts of the Integrated Review and there is no clarity over the Government’s strategic priorities.”