The number of men and women donating sperm and eggs could be halved as a result of the Government's decision to lift their right to anonymity.

A survey for the Department of Health found 50 per cent of donors said they would stop giving samples if they could be traced by their biological children. The head of the British Fertility Society, Richard Kennedy, said he believed the lifting of anonymity was "disastrous".

Children born from eggs or sperm donated after March this year will be able to trace their genetic parents. About 7,000 women a year receive fertility treatment using donated eggs or sperm and about 2,000 children are born.

Dr Kennedy said: "We realise that there is a strong case for children to know their genetic parents but the downside of that has led to a major reduction in the availability of donors. "

But Eric Blyth, the professor of social work at Huddersfield University who analysed the survey data, said: "I think everyone accepts donation rates are going to go down, but no one knows by how much. Personally, I think that is offset by the fact that the children ... have a fundamental right to know the identity of their genetic parents."

Another study by Professor Ian Craft, one of Britain's leading fertility experts, of more than 700 couples found that 60 per cent who had received donated material said they would not want the treatment if they knew that genetic parents could be traced.

The Government has launched a campaign to recruit more donors, including targeting people at blood donation clinics. The maximum payment that can be made to donors has also been raised from £55 to £250. The children of sperm and egg donors will not have any financial claim on their genetic parents.

* Pre-implantation genetic diagnosis, a controversial screening technique, could dramatically increase the success of fertility treatment, researchers in California have found. Even healthy donors produce a high number of eggs with chromosomal abnormalities, a study found.

Folic acid could stop miscarriages

Folic acid could help infertile women become pregnant and may reduce the risk of miscarriage, research has found.

Women are already told to take folic acid supplements while trying to conceive and in the early stages of pregnancy to reduce the risk of abnormalities such as spina bifida. But experts have now found the vitamin could also benefit women who have unexplained fertility problems or suffered repeated miscarriages.

Researchers from the West Texas Reproductive Centre studied 31 women, all of whom had a genetic mutation called Mthfr and had sufferedfertility problems or recurrent pregnancy loss. They were given a high dose of folate, or vitamin B9.

Eighteen subsequently became pregnant, a rate of 61 per cent, or six times higher than before the women were taking the supplements.