Cockle-pickers who lost their jobs at a west Wales fishery travelled to London on Friday to voice their protest.
Hundreds of livelihoods depend on cockle-picking
Their cockle beds were closed by the Food Standards Agency after tests showed the presence of a harmful toxin.
A group of more than 40 of the men gathered outside the agency's headquarters to express their anger at the closure.
They then intended to take their protest to No.10 Downing Street, where it was planned to hand in a petition.
The cocklers claim the agency's tests, which showed the presence of Diuretic Shellfish Poisoning, are not reliable.
The test involves injecting mice with a highly-concentrated mix of cockles and other chemicals.
But the cocklers argue that the human equivalent of this would mean someone eating 1.33 times their own body weight of cockles in one sitting.
Disputes over the test have led to a different method of analysis being introduced in Ireland, The Netherlands and New Zealand. But so far the FSA has refused to follow suit.
This is the second time in days that the Welsh cockle industry has been in the news.
Earlier this week hundreds of fishermen flocked to the Dee Estuary beds as restrictions imposed for conservation reasons were lifted for three days.
Tough controls are maintained to ensure that the number of cockles - an important source of food for wading birds - do not drop to an unacceptable level.
The Environment Agency issued more than 600 licences for cocklers at Thurstaston and Mostyn Bank in north Wales.
Some cockle beds are subject to restricted fishing
Cockle-pickers there have called for the beds to be open for longer periods, but under more strict regulation.
They say the beds mean a huge financial difference to the economy of the area.
Colin MacDonald who owns a cockle production company in Holywell, Flintshire argued that three days was not enough to harvest the number of cockles in the River Dee.
"I've got a cockle processing plant that's ready to go if need be but you can't run a business on three days of fishing," he said.
"The price of cockles is so high it could be a bonanza year."
But the Environment Agency says it must first ensure that cockle levels are high enough to allow harvesting without causing a threat to future stock levels.
If the tests show numbers are higher and the cockles are mature enough they are hoping to reopen the beds to the fisherman for a longer period at the end of August.
Meanwhile, in the north of England there have been complaints that gangs were coming in from as far afield as Malaysia, China and Europe to pillage the cockle beds.
In Morecambe, 37 suspected illegal immigrants were arrested for picking
cockles from a beach in the Chatsworth Road area.