Ricin, which was used to kill Georgi Markov, the Bulgarian dissident who died after a poisoned pellet was fired into his leg from the tip of an umbrella on Waterloo Bridge in 1978, is one of the strongest natural toxins in the world.
The necklaces, on sale for 75p, were produced in Guatemala from castor- oil seeds. About 7,000 were distributed to 346 Oxfam shops between 8 June and 17 June.
Each necklace was two feet long and contained more than 40 seeds.
A total of 1,151 necklaces had been sold before the charity was alerted to the danger. Advertisements are being placed in today's newspapers to warn the public.
Although the necklaces were intended to be worn by adults, Oxfam fears that the biggest danger is to children, who may suck or bite through the dangerous beads.
It has yet to trace a woman who bought 11 necklaces from the Oxfam shop in Shaftesbury, Dorset, and who told staff she planned to hide them around the garden as a game for a children's party.
Many of the cheap necklaces are likely to end up as children's toys, with the seeds becoming less easily recognised when separated from the necklace string.
The National Poisons Unit at London's Guy's Hospital has been told of the sale of the beads, which are being examined separately to assess their toxicity.
Dr John Henry, consultant physician at the unit, confirmed that the beads were highly dangerous. 'Swallowing one bead could kill a child,' he said.
Contact with a damaged seed is likely to cause a rash. Most adults would have to swallow several seeds to receive a fatal dose of poison, but other symptoms include vomiting and diarrhoea, as the poison is absorbed.
John Magrath, of Oxfam, said it was the first time the charity had recalled goods, adding that it had done everything it could to minimise the risk to the public. Announcements were being made on local radio in areas where large numbers of sales were known to have been made.
The danger was spotted by a botany student in Birmingham, who told the local Trading Standards Authority. By that time, sales had been made in virtually every county, ranging from two in Durham to 129 in London and 113 in Surrey. Necklaces had also been distributed and sold in Wales, Scotland and Northern Ireland.
All customers who have bought the beads are urged to return their purchases them to their local Oxfam shop immediately. Oxfam imported 10,000 of the necklaces, but only 7,000 were distributed.
Known as 'mother-in-law's poison' in India because of its ability to kill without extraordinary signs of illness, castor seeds are common in Central America and Africa.
The poison pellets used to murder Mr Markov were of refined ricin and were first tested on a horse and later on a condemned prisoner.
Mr Markov died in hospital four days after the incident on Waterloo Bridge in central London. He thought he had been bitten by an insect.Reuse content