Donation website The Big Give has been accused of making its 'double your donation' scheme too complicated and misleading charities.
The Reid Foundation, which set up The Big Give, has pledged to give £1.5m in match funding through the initiative next month.
To be eligible for the funding, a charity must have generated up to £50,000 from major donors and board members during phase one of the scheme in the summer.
The sum raised will be matched by the foundation only if the charity holds a second phase of fundraising in which new supporters give double the amount originally pledged.
So if a charity raised £50,000 in phase one, it must raise a further £100,000 in the second round before finally being awarded £50,000 from The Big Give.
Pauline Broomhead, chief executive of the Foundation for Social Improvement, a training organisation for small charities, described the scheme as "incredibly complicated".
Mind in York, for example, told donors their phase one fundraising would be matched, but did not realise a second round of fundraising was necessary, she said.
Jon Brooks, managing director of The Big Give, agreed the scheme was complex but said it was popular and that £1m awarded by the foundation in a similar scheme last year was snapped up in 45 minutes.
He said it used major donors in phase one as a catalyst for further fundraising and could potentially generate a total of up to £9m.