Retailers have been asked to ration sales of baby milk powder by the manufacturer of Britain's two most popular brands after evidence that the products are being bought in bulk for "unofficial export" to China, where demand is high for foreign-made milk.

Danone, which makes Aptamil and Cow & Gate – the market leaders in the UK – has asked supermarkets and chemists to limit purchases to just two 900g tins per purchase. Asda, Sainsbury's, Tesco and Morrisons have already agreed to limit purchases to two units per customer and more retailers are expected to follow suit.

Danone said the limit was to prevent some individuals from bulk buying baby milk for commercial purposes, for "unofficial exports". In some cases the milk is being sent abroad to relatives, but it is also being sold on the internet.

Foreign-produced baby formula is popular in China, where locally-produced and often contaminated products have resulted in deaths and widespread illness. The first episode of milk contamination in China occurred in 2008, when three of the top dairy firms in the country were proven to have been affected by the presence of melamine, an industrial chemical normally used in plastics. Six infants died and a further 300,000 fell ill.

The scandal-hit dairy industry has since shown no signs of improvement, forcing thousands of worried mothers to turn to foreign producers for baby milk powder. Online British food stores such as British Corner Shop have reported huge increases in business as a result.

Danone insisted that the rationing was highly unlikely to lead to shortages of its products, and apologised to parents for any inconvenience it had caused. It said in a statement: "We understand that the increased demand is being fuelled by unofficial exports to China to satisfy the needs of parents who want western brands for their babies. We would like to apologise to parents for any inconvenience caused by this limit. We know that most parents only buy one pack at a time, so we hope that the impact of this limit on UK parents will be minimal."

Morrisons said: "To maintain availability for all of our customers, we now have a limit of two items of formula milk powder per customer per day."

Richard Dodd, head of media and campaigns at the British Retail Consortium, said: "A number of retailers are limiting the amount of baby milk that can be bought by any one customer. This is being done at the request of the manufacturers, who believe it's possible that some organised groups of customers are buying up products in unusually large quantities for export. Retailers are taking this precautionary step to ensure stocks continue to be available to everyone wanting baby milk."