The figures come from voluntary sector think tank nfpSynergy's Charity Awareness Monitor, an annual survey of 1,000 people. Its latest poll was carried out in July last year.
The figure of 42 per cent is down from 51 per cent in the same survey in September 2006. It is the lowest figure since the survey began five years ago.
The research shows the level of trust in charities is lowest among people aged between 56 and 64, but rises again among the over-65s. The 16 to 24-year-old age group is the most positive, with 47 per cent saying they trust charities.
"The younger age group sees charities as a strong, belief-driven part of the world, which they can relate to," said Joe Saxton, co-founder of nfpSynergy.
The results show stark geographical differences. People in the north west have the most trust in charities (51 per cent); Yorkshire and the north east is the least trusting area (26 per cent). Those in lower socio-economic groups trust charities less than those with higher incomes.
The Scouts and Guides have bucked the sector-wide trend: trust in these charities remains at 50 per cent. Saxton put this down to the positive media coverage of the groups' centenary celebrations last year and the fact that many people do not think of them as charities in the traditional sense.
Twenty-two per cent of respondents said they trust the Fundraising Standards Board, even though the self-regulation scheme had only been up and running for five months when the survey was done. "It shows the public wants an organisation that monitors fundraising standards," said Saxton.
The research highlighted the need for a sector-wide communications strategy, said Saxton: "The main question is, whose job is it to work out what the reputation of charities should be? There are lots of potential players with the resources and expertise, but currently there's no communications strategy."