The WFP says hunger is the world's number one emergency

The game, FreeRice, tests the vocabulary of participants. For each click on a correct answer, the website donates money to buy 10 grains of rice.

Companies advertising on the website provide the money to the WFP to buy and distribute the rice.

FreeRice went online in early October and has now raised 1bn grains of rice.

That is enough rice to feed 50,000 people for one day, the WFP said on Friday.

'Viral marketing'

The head of the WFP, Josette Sheeran, said: "FreeRice really hits home how the web can be harnessed to raise awareness and funds for he world's number one emergency."

She said word of the game has spread with the help of internet bloggers and websites like Facebook and YouTube.

"The site is a viral marketing success story."

FreeRice is the invention of US online fundraising pioneer John Breen.