Suicides in Britain have fallen to an all time low - but it has little to do with increasing happiness. Killing yourself is getting harder, and the result is a big saving in lives.

A total of 4,331 people committed suicide in 2005, bringing the three-year rolling average to 8.5 deaths per 100,000, the lowest level since the Second World War.

The death rate has fallen by 7.6 per cent since the mid 1990s, largely because it has become more difficult for people to end it all. But the progress report, published by the National Institute for Mental Health yesterday, says the rate is not falling fast enough to meet the government target of a 20 per cent reduction by 2010.

Research shows that removing access to a method is one of the most effective ways of reducing suicides because the act is often impulsive. The introduction of catalytic converters for cars in the 1980s, which were made mandatory for all new vehicles in 1992, has had a dramatic impact on the suicide rate by reducing the toxicity of exhaust gases.

Motor gas poisoning had been one of the commonest methods of suicide, causing 508 deaths in 1979. Figures for 2005 show there were 219 deaths by this method.

Modern measures that are saving lives are the targeting of suicide hot spots such as Clifton suspension bridge in Bristol, Beachy Head in East Sussex, the bridge at junction 37 on the M1 near Barnsley and certain Tube stations such as Mile End which have attracted people in distress. The installation of barriers, nets and volunteer patrols has reduced the number of people who jump to their death.

The reduction in pack sizes of paracetamol to 16 tablets in supermarkets and garages has also cut deliberate overdoses. The prescription painkiller co-proxamol, associated with 300-400 fatalities a year, is being withdrawn.

The biggest improvements have been seen in prisons and in mental hospitals. Suicides in prison fell by 17 per cent last year and among patients in mental hospitals from 217 in 1997 to 154 in 2004.

Jenny Bywaters, a mental health adviser to the Department of Health, said removing access to methods was one of the most effective ways of preventing suicide. "For many people suicide is an impulsive act... If there is not an immediate method to hand, they will not persevere.

"In terms of what we can do to prevent suicide... improving access to mental health services is important but it is harder to get evidence for its effects."

One of the most striking changes in the past 30 years has been the suicide rate among young men aged 20 to 34, which more than doubled between 1970 and 2000 but has since fallen by more than 20 per cent, for reasons that are unexplained.

Dr Bywater said the declining suicide rate showed "quite good progress". "Whether this means more people are living lives of quiet misery I couldn't say."