The SNP’s Westminster leader demanded answers after it emerged the UK Government was considering an offer of tariff-free trading that could see Scottish farmers undercut by produce of lower environmental and welfare standards.
Speaking at Prime Minister’s Questions on Wednesday, Mr Blackford asked for a yes or no answer on whether the reports were true.
He said: “If reports of this Tory deal are true, farmers will lose their livelihoods, rural businesses will collapse and, ultimately, families will be driven off the land.
“And let's be very clear. If that happens, this UK Tory government will be solely responsible.
“So, for once Prime Minister, give a straight answer to these farming families who are living with this threat.
“Can you categorically rule out that your government is prepared to sign up to a trade deal that will, at any future point, grant tariff-free access to Australian lamb and beef?”
The Prime Minister did not directly answer the question, instead urging Mr Blackford to be “proud” of Scotland.
He said: “I don’t think he does justice to crofters, to farmers across the country.
“I think he grossly underestimates their ability to do great things with our free trade deals, to export Scottish beef around the world.
“Why doesn't he believe in what the people of Scotland can do? Why is he so frightened of free trade?
“I think it is a massive opportunity for Scotland and for the whole of the UK and he should seize it and be proud of it.”
The Ross, Skye and Lochaber MP claimed the answer would send a “real chill” across Scotland’s farming communities.
He continued: "This UK Government led the betrayal of Scottish fishing and now the Tories planning to throw our farmers and crofters under their Brexit bus.
“This morning, Martin Kennedy, president of the National Farmers Union Scotland, told ITV that farmers will feel ‘seriously betrayed’ by these proposals.
“This deal would be the final nail in the coffin for many Scottish farmers and crofters. It will end a way of life that has endured for generations.
“I know that many of the Prime Minister's Tory colleagues privately agree with me – and want him to pull back from this deal.
“So, will the Prime Minister finally listen, think again and ditch a deal that will send our farmers down under?”
Mr Johnson once again refused to comment on the specifics of the deal, instead accusing the SNP MP of wanting to be controlled by the EU.
He said: “He is grossly underestimating the ability of the people of this country, the agricultural communities and the farming industry to make the most of free trade.
“Our food exports are second to none and he should be proud of that, he should be celebrating that."
NFU Scotland on Tuesday contacted Scottish politicians to seek their support in ensuring that any trade deal with Australia followed the correct process.
Union president Martin Kennedy said he was concerned over the Australia FTA negotiations, stating that Scotland’s beef, dairy, sheep and grain sectors were particularly exposed in the event of a rushed deal.
The spat comes amid infighting over the deal, with the UK Government Cabinet split over a trade agreement.
Environment secretary George Eustice and Michael Gove are believed to be in disagreement with international trade secretary Liz Truss and Brexit minister Lord Frost over the shape of a deal.
Lib Dem MP Alistair Carmichael has now accused the Tories of being out of touch with the needs of farmers.
He said: “The idea is to ensure that keystone industries like farming are not undermined by one-sided negotiations, giving free access with nothing in return.
“Half the Cabinet, it appears, wants to sell farmers and crofters down the river to get a trade deal.
"Based on his responses in Parliament today, it appears that the Prime Minister has joined the anti-farming brigade.
"The Tories have lost touch with the needs of rural communities in their rush to undermine farmers and crofters – if they continue to take them for granted, then they may have a rude awakening in the next election.”