Doctors warned yesterday that fertility treatment for hundreds of couples could be threatened by a shortage of donated sperm, following government plans to let donor-conceived children trace their 'parents'.

Until now, male donors have remained anonymous and had no contact with any offspring. But Health Minister Melanie Johnson is expected this week to unveil a change in the law allowing children to request 'identifying information' about their donors when they turn 18.

Some fertility experts fear that men will stop coming forward for fear of being startled, decades later, by adult children turning up on their doorsteps. It is thought the measures, which will not affect those who have given sperm in the past, could also affect female donors of eggs.

Dr Ian Gibson, chair of the Commons Science and Technology Select Committee, said yesterday it would be investigating the impact of the changes. A major meeting of fertility experts is also to be held in March to debate them.

'We have got to come out clearly with a lot of answers to their questions,' said Gibson. 'It's all right when everybody is prepared to help medical science, but they are less ready now to do it than they used to be because they don't know what's going to happen.'

Donors will not be expected to take financial responsibility for supporting any children produced - about 1,000 a year are born following artificial insemination - and are expected to have the right to refuse to meet them.

However fertility expert Simon Fishel, of the Park Hospital in Nottingham, warned there could be a 'massive drop-off in available donors', most of whom are students. Such fears are likely to trigger renewed debate over whether sperm donors should be paid, in order to encourage recruitment.

Ministers have acted following a court ruling last year that donor anonymity infringed the rights of children to know their origins.