Angry cockle pickers could end up in jail if they continue defying a ban on collecting the shellfish.
The closure of the cockle beds has upset the workers
Last month, the Food Standards Agency (FSA) stopped workers from gathering cockles in Burry Port, south west Wales, because of safety fears.
The ban was introduced after shellfish collected in the area tested positive for a form of poisoning - and put 55 licensed cocklers out of work.
But on Tuesday, more than 35 of them - who say the tests that are used by the agency are unreliable - defied the ban.
If caught and prosecuted, they could face six months in prison or be handed a maximum fine of £20,000.
The cockling industry is an important part of the local economy, but the beds have been hit by a series of closures over the last two years.
Glyn Hyndman was among the pickers who broke the ban
The problem stems from a biotoxin called diuretic shellfish poisoning (DSP), which the FSA claims has been found in a sample of cockles from the area.
Contaminated cockles can lead to nausea, vomiting, abdominal pain, diarrhoea and chills.
Sue Davies, from Carmarthenshire County Council, said cocklers were breaking the law, even if they were only collecting shellfish for their own consumption.
"It is illegal - a temporary prohibition order is served for public protection," she added.
"We will be keeping a watch on the Burry Port area and collating any evidence we can from the activity."
The pickers are furious that the beds have been closed, saying it is crippling them financially and they have no choice but to go back to work.
"It's either bankruptcy or this - we have got to make a stand somewhere," said cockler Glyn Hyndman.
The workers have demanded the Food Standards Agency tests be changed.
"The FSA refuses to use other tests that are more accurate and which are the accepted methods for other countries such as Holland, Ireland and New Zealand," said Rory Parsons who runs a processing plant in Burry Port .
"The test is out-of-date - there are better methods."
But the FSA has stressed the test is EU approved.
"We've carefully considered the tests and have no doubts about the methodology used by our laboratories," said the FSA spokesman.
Carmarthenshire Council's head of public protection Phil Davies said there will need to be two consecutive negative tests before the beds can be re-opened.
"We cannot takes any chances when it comes to protecting public health," he said.