Simon Kalma, who lives in Golders Green, said he had noticed his local delis were no longer stocking kosher marmite, and contacted Unilever to find out why.
He said: "The ban is absolutely pathetic. It is inconveniencing people like me. It is not an illegal or banned substance. Putting aside business and legal reasons, this is discriminatory because they cannot provide me with Marmite in this country that is kosher."
Marmite manufactured in the UK used to be classed as kosher but lost its certification after Unilever changed the way it was manufactured. The South African version carries a hechsher from the Johannesburg Beth Din.
A British company called Jumbo Importers, which specialises in South African foods, has been importing kosher marmite for the past nine years. It supplies around 40 jars a month to stores such as Yarden, Kosher Kingdom and Kays.
Corne Bouwer, manager of the Slough-based firm, said he had received letters from Unilever telling him to stop the imports immediately.
"Because you can get marmite here you are not allowed to import the same product. It's all about infringement of the brand," he said.
"The kosher shops were disappointed but we have to obey the rules so we had to stop."
A spokesman for Unilever said: "The kosher marmite sourced from South Africa was being illegally sold in the UK and is no longer available for purchase. Marmite is 100 per cent vegetarian and, while there are no ingredients in the product which are not kosher compliant, due to the way it is heated in order for it to be decanted effectively, we cannot obtain kosher status."
The spokesman said Unilever had investigated the changes required to obtain kosher status from the London Beth Din but had decided against making them.
"They would involve substantial changes to the manufacturing process so it is not something we can do in the short term," he said.
But he did offer a slim hope to disappointed consumers by suggesting that the company was " looking into alternatives which may allow Marmite with kosher status to be available to purchase in the future".
But for the time being Mr Kalma and his fellow Marmite fans will have to find something else to spread on their bread.
"It's really annoying," Mr Kalma said. "My 14-year-old son takes Marmite sandwiches to school every day. They're his favourite."
