They "made the remarkable and unexpected discovery" that gastritis as well as peptic ulcer disease is the result of a stomach infection caused by the bacterium Helicobacter pylori, it said.

Marshall, 54, is from Kalgoorlie, Western Australia and researches at the QEII Medical Centre in Nedlands. Warren, 68, was born in Adelaide in South Australia and now lives in Perth where he worked as a pathologist at the Royal Perth Hospital until 1999.

"Thanks to the pioneering discovery by Marshall and Warren, peptic ulcer disease is no longer a chronic, frequently disabling condition, but a disease that can be cured by a short regimen of antibiotics and acid secretion inhibitors," the jury said.

Before the discovery of the bacterium in 1982, stress and lifestyle were considered the major causes of ulcers. But it has now been firmly established that Helicobacter pylori causes more than 90 percent of duodenal ulcers and up to 80 percent of gastric ulcers.

In rich countries, infection with the bacterium is less common than in the developing world "where virtually everyone may be infected", the Nobel citation said.

Last year, the award went to two American researchers, Richard Axel and Linda B. Buck, for pioneering work on our sense of smell, explaining how we recognise thousands of odours and remember them.

The Nobel prizes, founded by Swedish industrialist Alfred Nobel, the inventor of dynamite, were first awarded in 1901.

This year's laureates will receive a gold medal and share 10 million Swedish kronor (1.1 million euros, 1.3 million dollars).

The formal awarding of the prizes will take place at Stockholm's City Hall on December 10.

The Medicine Prize kicks off the coveted Nobel awards.

The Physics prize will be announced on Tuesday and Chemistry on Wednesday, and the Peace Prize on Friday. The Economics prize, awarded by Sweden's central bank, the Riksbank, is scheduled for October 10.

The Literature prize is traditionally awarded on a Thursday, though the actual date is only announced 48 hours in advance.