The board announced last week that the self-regulatory scheme will now begin in January 2007, three months after the planned date of 10 October.
So far, fewer than 100 organisations have joined. Of the UK's 10 biggest charities, the National Trust, the NSPCC, the RSPCA, the Salvation Army and the RNLI have committed to the scheme. Macmillan Cancer Support and Save the Children have both said they are about to join.
But Cancer Research UK, Britain's biggest charity, and the British Heart Foundation remain undecided.
Oxfam did not comment.
"I'd like to launch with 500 members," said Jon Scourse, the board's director. "Charities are joining at an increasing rate - I'm confident we can sign an additional 400 by January."
The board is paying marketing agency the Forster Company £250,000 a year to handle the public launch of the scheme, which was set up in April with £840,000 from the Government to improve fundraising standards.
Scourse said some charities had delayed joining because they did not want to sign up to a draft version of the Donors' Charter, which closed for consultation on 1 September. The final version will be published on 10 October.
Scourse added that charities investigated by the board following complaints from the public would be named. His chair, Colin Lloyd, had suggested charities would not be 'named and shamed'.
Judy Beard, director of communications and fundraising at Macmillan, said self-regulation was preferable to government intervention. "Imposed regulation could be a different proposition," she said. "The more of us that join, the better it will be for fundraising."