Sir Michael Fallon has accused the Labour leader of being "staggeringly irresponsible" on defence after the party descended into confusion over its stance on nuclear weapons.
Speaking on Sky News, the Defence Secretary said Jeremy Corbyn's "chaotic" approach to tackling terrorism and retaining the Trident nuclear deterrent would "put security of this country at risk immediately he was elected".
Sir Michael attacked Mr Corbyn for suggesting Labour would not commit to renewing the Trident nuclear deterrent, suggesting he would not launch a drone strike on the leader of Islamic State and saying he would suspend strikes against extremists in Iraq and Syria.
It follows an interview on Sunday in which the Labour leader refused to confirm whether or not renewal of Trident would be included, saying there was a "discussion" going on within the party.
Mr Corbyn - a lifelong opponent of nuclear weapons - also refused to give details as to what instructions he would give in his "letters of last resort", handed to those in charge of the country's four ballistic missile submarines.
But shortly after he made his comments, Labour said the party did still support the renewal of the nuclear deterrent.
On Monday morning Labour general election co-ordinator Andrew Gwynne told Sky News the party's commitment to Trident had been agreed at its conference and would be in the manifesto.
He said: "The Labour Party is committed to the Trident system."
But Sir Michael said the fact Labour had to come out to contradict Mr Corbyn showed the "chaos" in the party that further threatened the security of the nation, suggesting he was not fit to run the country.
He said: "I thought it was staggeringly irresponsible he has refused to support RAF strikes against terrorists, he is now questioning the nuclear deterrent, and we have already started building the new Trident submarines - and he seems to be at odds with his own party there, that's chaotic - and he is not going to get any kind of grip on immigration."
The Conservatives have already made clear they will focus their election campaigning on the Labour leader, branding him chaotic in contrast to Theresa May's "strong and stable".
As election campaigning entered its first full week on Monday morning, the Labour leader gave a speech to the Scottish TUC conference saying the party would "challenge the rigged system" and "stand for the many not the few".
He announced Labour would repeal the "vicious" Trade Union Act brought in by David Cameron, adding his party will "never, ever apologise" for its close ties to unions and he was proud to be a "card carrying" unionist.
Mr Corbyn told the delegates the trade union movement was the "driver of great progressive change in Scotland in the rest of the UK and across the world".
He also pledged to end zero hours contracts and protect the triple lock on pensions.
The Labour leader again denied that the election was a "foregone conclusion" and said the party was "campaigning to win every seat across the whole country".
Labour has just one MP in Scotland, Edinburgh South, and the latest polls suggest the party could be suffer a further hit, with support for Labour at 13% compared to 33% for the Conservatives and 44% for the SNP (Sunday Times Scotland poll.)
SNP leader Nicola Sturgeon will also speak at the TUC conference in Aviemore, warning that "hardliners have taken over the Tory Party" as she pitches the election as a straight fight between her party and the Conservatives.