Only one in three organisations will meet the site's accreditation criteria, she said.
From October, information on NHS Evidence will be accredited by a kitemark scheme designed to highlight the most trust- worthy pieces of evidence.
Rather than approving individual pieces of information, the site will approve providers of that information.
Dr Gillian Leng, chief operating officer of NHS Evidence, said that the process needed to be discerning and to select the best evidence.
'We won't be able to accredit everything,' she said. 'To be useful, accreditation needs to highlight the most relevant, the most trustworthy sources.
'That is probably going to be about 30 per cent, so there will be some organisations where their products are not accredited.'
A number of medical royal colleges have welcomed the plans to accredit information on NHS Evidence.
Comment below and tell us what you think