A third of people in the UK think going to war in Iraq was justified, but six in 10 believe it was a mistake, a BBC survey suggests.
55% of people polled said they felt Britain was less safe
When the war began four years ago, two-thirds of Britons backed involvement, but the poll shows a marked decrease in support.
Some 29% of 1,000 people polled by ICM across the UK said taking action against Iraq in 2003 was right.
The results emerged amid escalating sectarian violence in Iraq.
More than 130 British troops have died on operations in the country.
The survey also found that more than half of people questioned said they would distrust the British government if it said military action were needed elsewhere because a country posed a threat to national security.
While 51% said they would not trust the government in such circumstances, 32% said they would.
And more than half of those interviewed said they would support the use of British troops in further overseas missions such as disaster relief or stopping genocide.
Some 57% were in support of this kind of action even if the countries involved did not pose a direct threat to Britain's national security, while 24% opposed it.
And while 5% thought Britain was a safer place since the Iraq war, 55% said they felt the country was less safe.
Half also said the issue of the war was very or fairly important in their voting decisions.
A similar BBC/ICM poll in 2004 suggested that 46% of Britons were in support of the war and 48% thought British troops needed to remain in Iraq.
However, by 2005, a third of people wanted British troops to leave Iraq immediately, a survey for BBC Newsnight suggested.
Escalating violence and the deaths of serving British service personnel are widely seen to have dented support for the invasion.
The total number of UK troops killed in operations in Iraq now stands at 134 - 103 of whom were killed as a result of hostile action.
Nick Sparrow, from ICM, which conducted the research on behalf of the BBC, said: "Four years on from the war most people in the country have now come to the view that the United States and Britain were wrong to take military action against Iraq in 2003."
He added: "The poll does however suggest there would be support for the use of British troops to take part in overseas missions such as disaster relief or to stop genocide, even if the countries involved do not pose a direct threat to Britain's national security."
ICM interviewed a random sample of 1,019 adults across the country by telephone between 2 and 4 March this year.