Its rugged northern coastline is a favourite summer destination for Britons. But holidaymakers have been warned away from certain beaches in northern Brittany that have been swamped by tonnes of potentially lethal rotting green algae.

Hundreds of local residents and environmentalists demonstrated on the otherwise picturesque beach of St-Michel-en-Grève at the weekend as the "green menace" manifested itself in record piles of noxious seaweed swamping the shore.

A man has also taken legal action after he was left seriously ill from breathing in fumes from the decomposing algae. Vincent Petit, a 27-year-old vet, had to be dragged unconscious from a patch of rotting algae a metre deep this month after the horse he was riding collapsed and died from fumes given off by the sludge on the beach. The horse died within minutes.

Last year, two dogs died while walking near piles of algae on a beach close by.

Local mayors said more than 70 beaches had been hit by the seaweed that has plagued the coastline for more than a decade, but which this year reached unprecedented levels.

Some town halls in Brittany are spending more than €100,000 (£86,000) a year on bulldozing the seaweed away, but levels are still rising. The seaweed is harmless when it is alive, but as it decomposes on the beach – often releasing a foul stench of rotten eggs – the gases can be toxic. On the Côtes d'Armor coast, certain patches of shore have been closed and warnings have been issued to the public.

Environmentalists blame pollution from intensive farming. Brittany has a high concentration of pig, cattle and poultry farming and campaigners say nitrates from the farms are polluting the water system and feeding the prolific algae, which are then washed in to shore.

André Ollivro, vice-president of the campaign group Halte aux Marées Vertes (Stop the Green Slick), said: "Places where I used to sail, where my children used to build sandcastles on the Bay de Saint-Brieuc have disappeared under green algae and sediment. The solution is to stop pollution from intensive farming. It literally stinks inland because of places like pig farms – and now it stinks by the sea."

Local authorities said they had made efforts to minimise the level of agriculture in the area and to reduce quantities of farming refuse released into the sea.

The Centre for Study and Evaluation of Algae in Brittany said the problem was not "systematic" and tourists should not stay away from the whole region.

Potentially toxic algae has posed problems elsewhere in the past, including in Wales, where warning signs were put up this summer at a Snowdonia lake, near Llanberis in Gwynedd, warning lake users not to swim in the water or let animals drink it. The warning was later lifted after the council said tests showed that the algae blooms had subsided and that the lake was safe to use. The algae responsible for the problems in Wales, Anabaena spirioides, has a 50% chance of being toxic.

In New Zealand, an unidentified toxic algae was blamed this week for causing the deaths of two dogs on beaches in Auckland. Scientists who tested samples said the deaths were caused by a naturally occurring neurotoxin.