The farming industry in Wales may have to meet the cost of future outbreaks of animal diseases such as foot-and-mouth, without any government compensation.
Some politicians said the taxpayer should not pick up the bill
Farming union leaders have already attended government meetings to discuss the idea in principle but say the industry cannot afford the costs.
More than 360,000 animals were slaughtered in the 2001 foot-and-mouth epidemic in Wales, at a cost of £3bn.
The assembly government said it was looking at the proposed changes.
Farming leaders said they were worried about the implications.
"We're very, very concerned because the process is snowballing somewhat," said Gareth Vaughan, president of the Farmers Union of Wales (FUW).
"Initially we were talking about the cost of dealing with exotic disease only.
"But by today Defra are talking about the cost of dealing with animal health issues in total - and that the industry should pay for the lot.
"This is a total bill of something in the order £500m and our argument of course is that the industry cannot afford it, we just simply cannot afford it without having some measure of control as to what comes into this country."
Some politicians are adamant the taxpayer should not have to pay for future bills.
Labour MP for Newport West Paul Flynn said the government should not be "picking up the bill for everything that goes wrong".
He said: "Why on earth do we say that there's one industry that's so special that it's compensated for all its losses? It's a nonsense.
"Farming should do what other industries do and take out insurance.
"Many people suffered [in the foot-and-mouth outbreak]. The farmers had quite generous compensation in many cases but many other people in the leisure industry lost money and weren't compensated.
"I don't believe it's possible to have a policy where you say you give a free insurance policy to one industry and not to all the others," Mr Flynn added.
"Sadly the government couldn't give an insurance policy to those who lost money in the Farepak scandal."
A spokesperson for the assembly government said no decision had been taken and a consultation would begin next week.
"Both the industry and government have been working closely together in the run up to the consultation and will continue to do so throughout the process," he added.