Ian Blackford claimed the UK Government had thrown “Scotland’s interests under the Brexit bus” amid reports the Prime Minister is considering an offer of tariff-free trading that could see Scottish farmers undercut by produce of lower environmental and welfare standards.
The SNP Westminster leader said the move threatened the viability of many Scottish farms and could leave families in ruin.
He said: "Boris Johnson has utterly betrayed Scottish farmers and crofters with a post-Brexit trade deal that threatens the viability of Scottish lamb and beef farming, and could wreck a way of life that has existed across Scotland for generations.
"It is completely unacceptable that the Tory government is threatening to impose a deal that would undercut the price and standards of Scottish beef and lamb - putting Scottish farmers out of business.
"Yet again, Scotland's interests have been thrown under the Brexit bus - just as they were when the Tories sold out our fishing communities, and dragged Scotland out of the EU against our will.
"Brexit has already cost Scotland billions of pounds, and this Tory deal would be a disaster for our agricultural sector.
“There is a very real danger that many farmers will lose their livelihoods, rural businesses will collapse, and families will be driven off the land.”
The Prime Minister’s spokesman would not be drawn on details of the deal, and repeatedly refused to deny the UK would permit hormone-treated beef coming in as part of it.
He insisted: "Our priority will always be to ensure the best interests of British consumers."
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.
Today Home Secretary Priti Patel insisted the cabinet was united on the deal.
She said: "The government is united on every single level. We are working at every level to secure the best outcome for our country."
NFU Scotland on Tuesday contacted Scottish politicians to seek their support in ensuring that any trade deal with Australia followed the correct process.