Boris Johnson has batted away fears that an Australian trade deal could spell disaster for British farmers.
A furious Cabinet row has opened up over plans for a post- Brexit trade pact, which could include tariff-free access for Aussie farmers.
International Trade Secretary Liz Truss is said to be pushing for the deal, which would phase out the 20% tariff Australia pays on exports of beef to the UK over the next 15 years.
But Environment Secretary George Eustice is under pressure to protect British farmers from imports of cheap meat flooding British markets.
The move is said to have caused a furious row in Government, with allies of Ms Truss and Brexit Minister Lord Frost claiming their opponents are “more Waitrose than red wall”, according to Politico.
Mr Johnson signalled his support for the deal at Prime Minister's Questions where he said free trade was a "massive opportunity".
SNP Westminster leader Ian Blackford warned him that the mooted pact would be "disastrous" for farmers and urged the PM to rule out tariff-free access to lamb and beef.
"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," he said.
Bur Mr Johnson retorted: "I don't think that he does justice to crofters, to farmers across the country, and in Scotland as well.
"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 there is a massive opportunity for Scotland and the whole of the UK, and he should seize it and be proud of it."
Plaid Cymru's Hywel Williams also challenged the PM to rule out tariff-free deals for Australia, warning "it looks like he is backing Australian farmers instead" of Brits.
Mr Johnson said: "I will back Britain's farmers and Welsh farmers in exporting their fantastic lamb around the world.
"Is it not a disgrace that a single morsel of Welsh lamb has passed the lips of Americans in the last 20 years. What about China?
"Does he have no ambition? I do."
Ms Truss insisted British farmers would "not be undercut" by unfair practices from elsewhere.
She told the International Trade Committee: “This deal we are working on with Australia, we are currently in a sprint, with a view to getting to agreement in principle in early June.”
Asked about farmers, she said: “I have had discussions with NFU and I have been very clear with them that of course I am always looking to make sure - as I have committed to - that British farmers will not be undercut by unfair practices from elsewhere, and we will make sure in all the deals we will do that British farming thrives.
“I am absolutely confident that will be achieved through the Australia deal.”
Signing free trade deals around the world was touted as one of the major benefits of Brexit - and a deal with Australia would be highly prized.
It would be the first significant pact signed since the UK severed its ties with Brussels.
But zero-tariff trade could cause "irreversible damage" to farmers at home, critics have warned.
NFU president Minette Batters said: "We know that if we're to open up the opportunities of new markets overseas for UK farmers, we will have to offer greater access to our own markets in return.
"However, this trade-off needs to be balanced, and we need to make sure concessions to our hugely valuable home market are not given away lightly.
"There is a very real risk that, if we get it wrong, UK farming will suffer irreversible damage rather than flourish in the way we all desire, to the detriment of our environment, our food security and our rural communities."
