Organised criminals who send migrants on to beaches to illegally harvest shellfish could be filling a gap in the market caused by lockdown, a fisheries expert said.

Coastal authorities have reported gangs from around the country travelling to the seaside and flouting regulations set up to protect sea-life levels and food safety standards.

Police in Essex seized more than 810kg of shellfish from gangs of illegal pickers who were suspected of being forced to work by organised crime groups.

The haul, confiscated during one week by officers along Southend’s coastline, had an estimated retail value of £11,000 and included oysters, winkles and cockles.

Officers spoke to 29 pickers and many of them needed help from the charity Justice And Care, which rescues victims from slavery.

And in Redcar near Middlesbrough, dozens of people were stopped from illegally harvesting shellfish on a beach where cockles were unsafe for human consumption.

Police, marine life and labour abuse authorities carried out an operation on Tuesday following intelligence about large groups coming to the area.

Officers from North Eastern Inshore Fisheries and Conservation Authority found up to 40 people had harvested undersized crab and lobster on a commercial scale.

And on Wednesday, a family group was found to be picking cockles to use in a restaurant in the Northumbria Police area.

Cockles found in the Redcar and Cleveland area are not fit for human consumption and eating them could cause severe illness and be potentially fatal for children.

That is because the sea there has not been classified for bivalve consumption, so the bacteria levels in cockles is not known.

David McCandless, chief officer at the North Eastern Inshore Fisheries and Conservation Authority,said: “Our concern is that some of the individuals we have come across harvesting have connections with Chinese restaurants.

“They have been claiming these were just for personal consumption, but we were concerned they might not be.

“The individuals come from various parts of the country, there were people connected to Wales and West Yorkshire (in Redcar).

“Talking to my chief officer counterparts elsewhere, they have experienced very similar situations with groups just descending on areas to harvest shellfish.

“It seems to be a common issue around the English coast in many places.

“We don’t know if it is linked to Covid situation – up until recently markets have been closed where individuals could buy shellfish.

“It’s very unusual to come across this.”

He asked the public to be vigilant about large numbers of people gathering to harvest shellfish on beaches.

The Gangmasters and Labour Abuse Authority, which was set up following the Morecambe Bay tragedy in which 23 cockle pickers died in 2004, believed gangs were controlled by people linked to Chinese restaurants and takeaways, although this was unconfirmed.

GLAA head of enforcement Ian Waterfield said: “Ending exploitation and unsafe practices in gathering shellfish remains a top priority for our organisation, particularly because of the very real threat to life this work carries if it is conducted illegally.”