Kit Kat is a best-selling sweet around the world

The name of the chocolate bar resembles a Japanese expression - "kitto katsu" - used by students to wish each other luck before exams.

The phrase has been translated roughly as: "I hope you will win."

Kit Kat has introduced a range of flavours designed for the famously sweet-toothed Japanese market, including green tea flavour.

Other variations include passion fruit, white chocolate, and lemon cheesecake.

Determined pupils

The multinational food company Nestle, which makes the bar, said it had noticed a surge in demand.

"We're finding that parents are buying them for their children for exam days," Yuko Iwasaki, a spokesman for Nestle Japan, told Britain's Daily Telegraph newspaper.

"But also some determined pupils are buying Kit Kats for themselves as a sort of reminder that they are really going to give these exams their best shot," he added.

The Kit Kat was invented by Britain's Rowntree confectionery company back in 1935, but until 1937 was called the Chocolate Crisp.

The Kit Kat name is thought to derive from a club of the same name in 1920s London.

It has long been the most popular chocolate bar in the UK.