Theresa May has declined to promise Wales will not lose any money it now receives from the EU after Brexit.
The prime minister reiterated her government's commitment to maintain EU funding levels of around £300m a year for Welsh agriculture until 2022.
Pressed on funding beyond that, she said: "We'll be making sure that we get a system that is in the interests of farmers across the United Kingdom."
Mrs May was speaking to BBC Wales as she visited the Royal Welsh Show.
At the event in Llanelwedd on Thursday, the Farmers' Union of Wales warned the prime minister rural Wales would "break down" without a Brexit deal with the EU.
Meat Promotion Wales claimed on Monday that a no-deal Brexit would cause a "seismic shock" throughout the Welsh farming industry.
Mrs May said ministers in Westminster were making "sensible" preparations for a "no deal" Brexit, whilst also working for a "good deal" with the EU.
When asked whether she was prepared to take such a risk with the Welsh economy, Mrs May said: "We're working to get a good deal for our future relationship with the European Union - that's why we've published the white paper that's based on the Chequers agreement.
"That sets out opportunities for us to continue to have good trade with the European Union in the future whilst delivering on what people for.
"We'll prepare for a 'no deal' because it's sensible, practical, to make those pragmatic preparations.
"But what we're working for as a government is to get a good deal, and a good deal based on the Chequers agreement which, as I've heard today, would be a good deal for Welsh farmers."
On funding, Mrs May said: "Well, first of all we've made a commitment to farmers. We've said that the overall funding will be the same as under the CAP [EU Common Agricultural policy] until the end of this Parliament.
"We've actually given farming that certainty up until 2022. That enables people to make the planning necessary.
"And we will be making sure that we get a system that is in the interests of Welsh farmers, in the interest of farmers across the United Kingdom.
"What matters is that out of the EU we'll be able to put in place arrangements that are in our interests and the interests of our producers and not part of the Common Agricultural Policy."
Welsh Government Rural Affairs Secretary Lesley Griffiths said: "The best thing the prime minister can do to help us to deliver our responsibilities for the Welsh farming industry is to avoid a hard Brexit and deliver on the promise made during the referendum that Wales won't lose a penny of funding as a result of leaving the EU."
'Promise lies in tatters'
Plaid Cymru MP Jonathan Edwards said EU funds had been a "lifeline" for Welsh farming and the industry would be "far better off staying within European frameworks".
"In the run-up to the EU referendum we were promised that rural Wales would not be a penny worse off after Brexit," he said.
"After today's deeply worrying comments by the prime minister, that promise lies in tatters."