There could be "civil unrest" in rural Wales if the next prime minister takes a no-deal Brexit, it has been claimed.
The Farmers' Union of Wales (FUW) predicted protests, while NFU Cymru said it would not rule out campaigning to stop Brexit entirely.
Meat Promotion Wales said the impact of no deal would be "off the Richter scale".
UK Environment Secretary Michael Gove said they should focus on persuading MPs to support an "orderly" Brexit.
Mr Gove also said he wanted to stay as environment secretary under either Boris Johnson or Jeremy Hunt following the result of the Conservative Party leadership election, which is announced tomorrow.
They were speaking at the start of the 100th Royal Welsh Agricultural Show in Llanelwedd, Powys.
Both Mr Johnson and Mr Hunt have made it clear they would take the UK out of the EU without a deal, if necessary.
But there was "a possibility of some civil unrest" if that happens, FUW president Glyn Roberts said.
"If the farming community have their backs against the wall, the only way they're going to get from there is fighting their way through," he added.
NFU Cymru president John Davies said the organisation would "rule nothing out" in avoiding a no-deal Brexit.
He criticised politicians who had been "quick to dismiss our well-founded concerns as scaremongering".
Others argued leaving without a deal at the end of October was "the worst possible timing for farmers".
A new report, commissioned by the Agriculture and Horticulture Development Board (AHDB), Quality Meat Scotland and Meat Promotion Wales suggests exports to Europe, which is currently the destination for more than nine-tenths of overseas trade for Welsh producers, could fall by about 92%.
The report estimates this would lead to a 24% fall in the price of lamb at a time of year when thousands of lambs are coming on to the market every day.
Meat Promotion Wales' chairman Kevin Roberts said research showed farming leaders would have to do all they can to avoid this scenario.
"We need a seamless and stable solution," he added.
Mr Gove said: "I've been very clear that if we have a no deal situation there will be unique challenges... We in the UK government are ready for any eventuality - we've taken steps to ensure that we have contingencies in place.
"Our aim as a government is to ensure we have a good deal.
"I think it's... important for us in UK government to maintain our perspective and calmness - we believe that a deal is possible, overwhelmingly likely - and therefore we have to ensure that we keep a focus on getting that deal and securing benefits of being outside EU."
On financial support for farms, he said there would be no way that "Wales will be anything other than a stronger position in the future".
The prospect of the UK leaving without a deal is an "increasing probability" which is causing "a great deal of unrest", Welsh Government Environment Minister Lesley Griffiths said.
"We've said all along that there has to be a deal. We thought we were leaving at the end of March, people geared up for that and now we're asking companies to gear up again for the end of October," she added.
"It's so much uncertainty and clearly people are really very concerned."
'You haven't aged a bit'
Also attending the show was the Prince of Wales and the Duchess of Cornwall.
They joined the King and Queen of Zulu, marking the 140th anniversary of the Anglo Zulu War. Charles was also celebrating his 50th anniversary as the Prince of Wales.
Charles, Camilla, King Goodwill Zwelithini and Queen Pumi were treated to a military display by members of a Zulu "impi" regiment, dressed in traditional leopard skin uniform.
The prince remarked to the Zulu King: "You haven't aged a bit." They last met in South Africa in November 2011.