An increasing proportion of Britons favours a more liberal approach to drugs and would support decriminalisation strategies, according to a comprehensive survey commissioned by the Observer.

An overwhelming majority also believes that the so-called "war on drugs" is futile, with 84% saying that the decades-long campaign by law enforcement agencies against the global narcotics trade can never be won.

The poll provides welcome reading for those campaigning for illegal drugs to be decriminalised, with 27% saying that Britain's drug laws are not liberal enough. A previous Observer survey into the nation's drug-taking habits, in 2008, recorded a figure of 18%, suggesting a society that is steadily moving towards greater tolerance of drug use.

The proportion of Britons who believe certain drugs should be decriminalised has risen from 27% to 39% since 2008.

More than half (52%) support the introduction of initiatives like that recently pioneered by two US states, Colorado and Washington. Colorado's decision to legalise the sale of recreational marijuana has been hailed a success by some, with reductions of crime reported in the state capital of Denver and concerns about social breakdown yet to be borne out.

In the UK, however, there appears to be little appetite among Tories for a fresh look at drugs policy despite David Cameron, as a young MP, endorsing more lenient penalties for ecstasy possession and formerly sitting on a parliamentary committee that called for an international debate on the legalisation of drugs. The Liberal Democrats are currently examining the decriminalisation of all drugs for personal use and allowing cannabis to be sold on the open market. This week the party will discuss a policy paper advocating such options at its annual conference.

Prohibition has failed to curb the popularity of narcotics, as the number of Britons who have taken drugs continues to increase. Almost a third of the adult population – up from 27% in 2008 to 31% now – say they have taken an illegal substance – about 15 million people. While men and women are equally likely to have taken drugs, those aged 35-44 are the most likely to have used narcotics, with almost half this age group having taken them.

Across all age ranges, around three million people continue to take drugs, half of whom are aged 16-34.

If drugs were decriminalised, however, the proportion of Britons who have never previously tried drugs but who would consider doing so in the future would increase fourfold to 16%, offering some proof to hardliners that drug laws act as a deterrent.

The effect would be most pronounced among young people. Among 16- to 24-year-olds, 30% of those who have never taken drugs say they would consider doing so if substances were decriminalised.

The recession appears to have a had an impact on drug consumption. In the 2008 poll, conducted towards the beginning of the global economic slump, 35% of users were more likely to use drugs in a pub/club/bar environment. This has now fallen to 16%, possibly an indication of more straitened circumstances. Users spend an average of £74.36 on drugs each month, compared with the £54.58 an average drinker spends on alcohol a month or the £76.73 a smoker spends on tobacco.

Concerns that legal highs would create an explosion in drug use have yet to appear, with only one in 10 Britons saying they had tried them. Among those aged 25-34 the proportion to have tried legal highs almost doubles to 19%.