Cadbury's famous glass and a half is becoming a millstone. The company has faced weeks of bad news: a £1 million fine for the national salmonella outbreak; a mass recall of mistakenly labelled Easter eggs, which clipped a percentage point off its share of the UK confectionery market; the delay of the £7 billion sale of its US drinks arm, and the loss this morning of £760 million in value. Alongside all this, Cadbury has said that the rising price of its key ingredient – milk – makes "market progression" unlikely this year.

Cadbury's chocolate has not always contained milk, but after the company started adding it to products in the early 20th century a future as market leader and the first name in chocolate followed.

When a young Quaker called John Cadbury opened his first shop in Birmingham in 1824 – notable for its head-turning plate-glass window and the Chinaman in full national costume who presided over counter – the "chocolate nibs" he sold alongside tea and coffee were made up with hot water or milk as drinking chocolate.

At the time only plain eating chocolate was available; there was no milk chocolate until 1875, when Daniel Peters of Vevey, a Swiss manufacturer, produced a bar combining cocoa and powdered milk.

In 1897 Cadbury was selling milk chocolate bars made with powdered milk, but then Peters edged ahead, replacing powdered milk with condensed. Swiss milk chocolate and "fancy" French chocolates led the British market.

The Cadburys responded by diverting money and years of research towards their endeavour to create a milk chocolate "not merely as good as, but better than" its continental rivals. Into every half pound of chocolate – so the advert promised – went the glass and a half of full-cream milk, and in 1905 Cadbury's Dairy Milk was launched. By 1913 it was Cadbury's best-selling line; by the mid 1920s it was the brand leader, and it has retained the position ever since.

The Milk Tray man may have come and gone, but the glass and a half remains: Cadbury says the recipe for its most famous chocolate bar is little changed since 1905.

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