The outcome will affect manufacturers including Mondelez International Inc's Cadbury and Nestle SA, which took over Rowntree in 1988.
Arguments over "common definition" will sit alongside tariff barriers and customs controls as obstacles to British food and drink manufacturers if Britain leaves the EU single market, Clegg said as he introduced a report on the UK's 27 billion pound (NZ$46 billion) food and drink sector.
The EU accounts for more than two-thirds of exports and 94 per cent when EU Free Trade Agreements with other countries are included, Clegg said, citing data from the Food and Drink Federation.
By having a single definition of chocolate it meant our chocolate manufacturers in York, the Midlands and elsewhere could sell chocolate to sweet shops in Spain, Greece and Finland just as effortlessly as they can in the United Kingdom.
"Food and drink products are currently traded across borders with no forms or checks, once we leave the EU, products will have to go through customs checks at the EU border which include applying for relevant import licences, costly export health certificates to show that the product meets EU public health standards, and veterinary inspections," Clegg said.
"Major markets for key exports like whisky, which brings in 4 billion pounds a year, will become more challenging as tariff-free EU-negotiated FTAs fall away."
Businesses and investors have expressed concern that Prime Minister Theresa May is planning a so-called hard Brexit, in which the government prioritizes immigration controls ahead of the ability to trade goods and services freely in the EU's single market.
Prices for consumers in the UK will rise if May opts for that approach and tariffs and the effects of currency fluctuations are passed on, Clegg said.
If the UK adopts World Trade Organisation terms on leaving the bloc, agricultural exports to the EU, worth 11 billion pounds a year, will be hit by average tariffs of 22.3 per cent, including 59 per cent on beef, Clegg said. Similar charges would be levied on imports under WTO rules, he said.
"The recent drop in the value of the pound and its impact on sales of Marmite and PG Tips is only a foretaste of what is to come in the next 12 months," he said.