Boris Johnson has said post-Brexit trade deals provide a “massive opportunity” for UK farmers – rejecting a claim from the SNP that a tariff-free agreement with Australia would see farmers “lose their livelihoods”.

The government hopes a deal with Australia will be the first major post-Brexit free trade agreement with a country that the UK did not have an existing trade deal with as an EU member state.

But there is understood to be division among ministers over the terms of the deal, with some concerned a tariff and quota free agreement could leave farmers struggling to compete.

They fear this could fuel pro-independence arguments in Wales and Scotland and cause further tension and disruption in Northern Ireland, where the protocol negotiated as part of the UK-EU Brexit deal requires checks on some imported food products to ensure they meet EU food standards.

The Department for International Trade has insisted any trade deals signed will not “undercut UK farmers or compromise our high standards”.

But speaking during Prime Minister’s Questions, SNP Westminster leader Ian Blackford asked the prime minister to “categorically rule out” signing an agreement that would remove tariffs and quotas on imported Australian lamb and beef products.

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.”

Mr Johnson responded by accusing the SNP leader of lacking confidence in Scottish farmers.

“He grossly underestimates their ability to do great things with our free trade deals, to export Scottish beef around the world,” he said. “Why doesn’t he believe in what the people of Scotland can do? Why is he so frightened of free trade?

“I think there’s a massive opportunity for Scotland and for the whole of the UK and he should seize it and be proud of it.”

The prime minister gave a similar response to a question from Plaid Cymru’s Hywel Williams, accusing him of having “no ambition for Welsh farmers”.

Earlier in the day, Labour’s shadow environment secretary Luke Pollard posted on social media: “It looks like the Conservatives are about to betray our British farmers over the trade deal with Australia in the same way they betrayed the British fishing industry over the Brexit deal.”

Minette Batters, president of the National Farmers’ Union of England and Wales, said her organisation does not oppose a deal in principle, but wants protections in place to ensure UK producers do not face damaging price falls.

Speaking to Sky’s business presenter Ian King, she said it was important to have the right measures in place to set precedent for future trade deals that unrestricted access to the UK market was not on the table.

“We’re not anti a trade deal with Australia. We think it’s absolutely part of the global Britain agenda,” Minette Batters said.

“We want to just have some checks and balances in place to make sure that farmers in this country are not undermined, which has of course been a cast iron commitment by the prime minister and this government.”

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