Although the research team did not look at any direct links between the MMR (measles, mumps and rubella) vaccine and IBD, their discovery could re-open the debate over whether the vaccine could be responsible for some cases.

The vaccine "infects" children with a weakened form of the live viruses to encourage an immune response which protects against the real thing.

So immunised children do come into contact with a much less powerful form of the two diseases in close succession.

However, the deafblind charity Sense urged parents to continue to immunise their children with the MMR vaccine.

They warned that failure to do so could lead to more cases of rubella (German measles) and a rise in the number of children born disabled as a result.

The discovery of the link by a team from the Royal Free Hospital in London has been hailed as a "major step forward" in finding a cure for incurable inflammatory bowel diseases (IBDs) such as Crohn's Disease and ulcerative colitis.

Another team from the Royal Free opened the original MMR controversy with reports published in the Lancet in 1997 which linked the MMR vaccine to IBD.

The Royal Free team studied data from 16,000 people all born in one week in 1970.

They found children aged four to six who contracted measles and mumps in the same year were four times more likely to develop Crohn's Disease, and seven times more likely to develop ulcerative colitis.

Crohn's disease has become five times more common in young adults since the 1970s, and while it is treatable it is not curable.

Team member Dr Scott Montgomery said: "This is really a very major step as until now we have had very little idea what causes these diseases.

"Now we have an idea about the causes we can work towards better treatments and a cure."

Difficult to detect

Dr Montgomery added: "With diseases like Crohn's, there can often be a long gap between the infection, like measles, and the onset of symptoms of IBD, which makes the cause more difficult to detect.

"We now need to do more work on this. More people are developing Crohn's and other IBDs than in the 1970s, so we need to start looking at possible cures."

Dr Montgomery said the combined impact of two childhood infections in rapid succession might prompt the immune system to turn against itself, leading in the long term to the development of IBDs.

Both measles and mumps are known to reduce the strength of the immune system.

IBDs are painful diseases affecting the intestine. Sufferers often spend their lives going through periods of remission followed by relapse.

Crohn's disease usually causes ulcers (open sores) all along the length of the small and large intestines. It may also cause inflammation or infection around the rectum.

Ulcerative colitis usually causes ulcers in the lower part of the large intestine, often starting at the rectum.

Both Crohn's disease and ulcerative colitis can be controlled by drugs and some patients have to undergo surgery to remove parts of the affected intestine.