Canada's bloody annual slaughter . . . the most controversial for decades . . . takes place as calls mount for a boycott of the country's products.

Hunters and protesters are heading for the Gulf of St Lawrence and the northeast coast of Newfoundland, waiting for the Canadian government to give the goahead for the cull to begin. Ministers have already authorised the slaughter of 325,000 baby harp seals, the second-highest number ever. It will be the third successive year in which more than 300,000 of the cubs have been clubbed and shot; by the end of the cull, the death toll since 2004 will top a million.

But the cull will take place in the face of the most determined attempt to stop it in more than 20 years. Attempts to launch a global boycott against Canadian exports started in Britain last week. Major supermarkets will tomorrow receive letters urging them to stop stocking Canadian produce, and vigils will start outside travel agents in 20 British cities to try to persuade people not to holiday in the country.

The supermarket campaign is being led by Sally Stratford, widow of the former Labour minister, Tony Banks, who was an ardent opponent of the cull. Ann Widdecombe, the former Tory minister, has also written to retailers to urge a boycott, and 188 British MPs have signed an early day motion to support it.

Paul McCartney and his wife, Heather, travelled to the floes this month to call for the cull to be called off. His designer daughter, Stella, is donating part of the proceeds from selling a special t-shirt to the campaign.

The boycott began last year in the US, supported by more than 400 restaurants, supermarkets and seafood wholesalers. This year it is expected to spread to France, Germany, Spain, Denmark, Mexico, Japan and the Netherlands.

The protesters are hoping to repeat a boycott of the early 1980s, which pushed Canada into banning the killing of the youngest pups, called whitecoats for the colour of their fur. Hunters now evade the ban by waiting a few days until the seals begin to turn grey.

Canada is vulnerable to a boycott because it exports 3bn worth of seafood to the United States and 110 million worth to Britain every year. This far outweighs the 12m value of the skins and other products sold from the hunt.

The Humane Society of the US, the country's leading animal protection charity, claims that the value of Canadian snow crab imports has dropped by 120 million since the boycott began last year, adding: "It is clearly having an impact".

Despite the killing, harp seals are not now an endangered species, but scientists say global warming could imperil them. However, the retreating ice may also signal the death of the cull. The government says water temperatures off Newfoundland are 4.5 degrees centigrade warmer than this time last year and that the ice is already beginning to melt. As a result, it says, hunters will not be able to get close enough to many of the pups to club them, and will have to shoot them instead.