Ask Charity, which will be password-protected to prevent it being used by non-media bodies, will supply journalists with the mobile phone numbers of voluntary sector contacts so queries can be answered at any time of the day.
It has been set up by the Voluntary Action Media Unit, which has a £457,500 Big Lottery Fund grant over three years to research and improve relations between the voluntary sector and the media.
The site is expected to contain details of about 5,000 organisations by the time of its launch, when it will be intensively marketed to national broadcasters, newspapers and magazines.
The second phase will include regional and local charities and will promote the service to local media organisations. It could eventually hold details of up to 20,000 voluntary groups.
Penelope Gibbs, director of the Voluntary Action Media Unit, said it was a common complaint from media organisations that charities were difficult to get in touch with.
"Big charities get the lion's share of media contacts because they are at the top of people's minds," she said. "Smaller charities, which may have the expertise the media needs, are often ignored. In future, journalists will be able to search for charities in a particular field."
She said she was considering making the inclusion of mobile phone numbers a condition of being listed on Ask Charity. "It really is the test of how willing a charity is to be responsive to media requests," she added.
"The sector can't say it wants to be accessible and then deny the tools that give access."
Sophie Livingstone, head of communications at the Foyer Federation, said she thought the idea was great in principle but would have to be heavily marketed to work in practice. "Journalists do tend to fall back on personal contacts," she said.
For more information on Ask Charity and to be included, go to www.vamu.org.uk/mediacontacts.