The research was carried out by the World Health Organization Collaborating Centre for Nutrition , based in the Division of Mental Health & Wellbeing of Warwick Medical School at the University of Warwick.

The study looked at the geographical distribution of habitual dietary salt intake in Britain and its association with manual occupations and educational attainments, both indicators of socio-economic position and key determinants of health.

The researchers used the British National Diet and Nutrition Survey (2000-1), a national representative sample of 2,105 men and women aged 19-64 years living in Britain. Salt intake was assessed with two independent methods: a 7-day dietary record and the 'gold standard' 24h urine collections for sodium determination (direct marker of salt intake).

The study provides evidence for the first time that salt intake is significantly higher in those with low educational attainment and in manual occupations, when the effects of geographical variations are stripped out (people living in Scotland had higher salt intake than those in England & Wales).

Professor Francesco Cappuccio, senior author and Director of the WHO Collaborating Centre, said: "These results are important as they explain in part why people of low socio-economic background are more likely to develop high blood pressure (hypertension) and to suffer disproportionately from strokes, heart attacks and renal failure."

Ms Teresa Morris of The Bupa Foundation, which funded the study, said: "Habitual salt intake in most adult populations around the world exceeds 10 g per day and the World Health Organization recommends that daily intake should not exceed 5 g .

"Population salt reduction programmes are a cost-effective way of reducing the burden of cardiovascular disease nationally and globally."

Professor Cappuccio continued: "We have seen a reduction in salt intake in Britain from 9.5 to 8.1 g per day in the period 2004-2011, thanks to an effective policy which included awareness campaigns, food reformulation and monitoring.

"Whilst this is an achievement to celebrate, our results suggest the presence of social inequalities in levels of salt intake that would underestimate the health risks in people who are worse off -- and these are the people who need prevention most.

"The diet of disadvantaged socio-economic groups tends to be made up of low-quality, salt-dense, high-fat, high-calorie unhealthy cheap foods.

"Behavioural approaches to healthy eating are unlikely to bring about the changes necessary to halt the cardiovascular epidemic and would also widen inequalities.

"Since the majority of dietary salt is added during commercial food production, widespread and continued food reformulation is necessary through both voluntary as well as regulatory means to make sure that salt reduction is achieved across all socio-economic groups," Professor Cappuccio concluded.