A majority of voters still believe that the Conservatives are the party of the rich, giving strength to the belief that David Cameron's "detoxification" campaign has failed, according to a poll.

The survey, by ComRes for the Independent newspaper, shows 51% agree with the statement, "The Conservative Party only represents the interests of the rich". Forty-two percent disagree.

The result followed claims by Conservative moderniser Nick Boles that the "party of the rich" image was the single biggest problem facing the party.

The prime minister rejected Boles' analysis and persisted in claiming that the party stood for "hardworking people" across the nation.

But the poll suggests he still has work to do to persuade voters that that is true. Possibly more worrying for the Tories is the suggestion that 54% of women agreed with the statement compared to 47% of men, highlighting the continuing problem Cameron has appealing to women voters.

Younger voters and those living in the north of England and in Scotland also are more likely to agree with the majority findings, although even in the traditionally Tory strongholds in the southeast, 48% agree while 45% disagree.

The survey result may be further evidence that Labour is on to something with its attempts to paint the Tories as the party exclusively of the rich and powerful.

But the poll also contains bad news for Ed Miliband. Just 33% of all voters trust him to protect the NHS compared to 30% for Cameron.

The figure seems to confirm recent indications that the old idea that the NHS was a given positive for Labour was being whittled away, particularly among Labour voters where 61% trust Miliband on the issue compared to 68% of Tories who trust Cameron.

Overall, the poll gives Labour a five-point lead over the Conservatives, down from eight points a month ago. Labour is on 37% (up one point on last month); the Tories on 32% (up four points); the UK Independence Party on 11% (down one point); the Liberal Democrats on 9% (down two points) and other parties on 11% (down two points).

Other recent polls, however, have given Labour anywhere between six and nine-point leads.