Eating less salt can cut cardiovascular disease risk by a quarter and fatal heart disease by a fifth, work shows.
The ideal daily intake of salt is no more than six grams and ministers want everyone to achieve this by 2010.
Experts already know that too much salt can raise blood pressure and high blood pressure increases the risk of heart attack and stroke.
The British Medical Journal study now gives the evidence behind this link and quantifies how much harm salt can do.
People who significantly cut back on the amount of salt in their diet reduced their chances of developing cardiovascular disease by 25% over the following 10 to 15 years.
And their risk of dying from cardiovascular disease went down by 20%.
All of the 3,126 people studied by the US team from Boston had had high-normal blood pressure, or "pre-hypertension".
In the trials, participants reduced their salt (sodium) intake by about 25% - 35%, from about 10g to around 7g.
And those who cut back tended to stick to a lower salt diet in the long term, the researchers from Brigham and Women's Hospital found.
Professor Graham MacGregor, a consultant in cardiovascular medicine at London's St George's hospital and chairman of the Consensus Action Group on Salt, said: "This is a very important study.
"It shows that if people reduce their salt intake it will reduce the number of people suffering from heart attacks, strokes and heart failure. We did not have that type of evidence before.
RECOMMENDED SALT LIMITS
1 to 3 years - 2 g salt a day (0.8g sodium)
4 to 6 years - 3g salt a day (1.2g sodium)
7 to 10 years - 5g salt a day (2g sodium)
11 and over - 6g salt a day (2.5g sodium)
Source: Food Standards Agency
"And we are only talking about quite small reductions in salt intake to have a big effect on risk."
Three-quarters of the salt we eat is already in the food we buy. The average daily consumption in the UK is 9g.
Professor MacGregor said the 6g target was achievable for most people if they were careful about the food they chose to eat.
He said the onus was on food manufacturers to limit the amount of salt in products.
Sodium is usually listed in the nutritional information on food labels, and multiplying this value by 2.5 will give the salt content.
The Food Standards Agency said it would continue its work in encouraging industry to offer consumers healthier choices.
Ellen Mason, cardiac nurse at the British Heart Foundation, advised: "By simply checking the labels and switching to a lower salt option, you'll be doing your heart a favour."
The Salt Manufacturers Association said the evidence did not prove that salt reduction would have any significant health benefits for the majority of people.
It conceded that individuals with high blood pressure might be advised to restrict their intake.