Researchers said new laws in the UK and Netherlands banning anonymity could have a negative impact on poor couples.

In Britain, all children conceived via donors from April will be able to obtain information when they turn 18.

The Leiden University Hospital study of 105 couples comes as UK ministers launched a campaign for more donors.

Critics fear the change will lead to a fall in donors as they will not want to be identified.

'Huge benefit'

Currently just 250 men donate sperm and 1,100 women donate their eggs.

Ministers said around 500 sperm donors and 1,500 egg donors were needed each year.

Posters, leaflets and business cards will be distributed to 95 fertility clinics saying "Give Life, Give Hope" during the campaign.

Health Minister Melanie Johnson said she hoped it would increase the number of donors as well as "raise awareness of the huge benefits" donation can bring.

Ms Johnson told the BBC's Radio 4 Today programme: "Many people would be happy to come forward but have never thought about it and how it can give life and give hope to infertile couples."

Dr Anne Brewaeys, the lead researcher of the Dutch study of 105 heterosexual and lesbian couples, said her findings illustrated education campaigns were needed to prevent "further stigmatisation of infertility".

Her team found 40 of the 64 heterosexual couples chose an identifiable donor, while 40 or the 41 lesbian couples did, the Human Reproduction journal reported.

Of the 39% of heterosexual couples to want an anonymous donor, the majority were from poorer social backgrounds.

She said the association between donor choices, education levels and infertility distress were "intriguing".

And she added the findings were likely to be because male fertility and non-genetic parenthood "remained more of a taboo" in poorer social groups.

Donor anonymity was banned in the Netherlands last year.

In a separate UK study, also published in the journal, 61% of 46 couples with children conceived through donor insemination were against explaining the situation to their children.

'Accidental discovery'

Some 13% of the couples, who had children between the ages of four and eight, had already told them, while a further 26% intended to in the future.

The two most important reasons for being open were avoiding accidental discovery and because parents wanted to be honest, researchers at the Family and Child Psychology Research Centre of London's City University found.

Lead researcher Dr Emma Lycett said while most couples still favoured non-disclosure, the climate was changing.

"I think there is a move towards more openness. Part of the reason is that the stigma surround fertility treatment is reducing."

Dr Allan Pacey, of the British Fertility Society, welcomed the government campaign but said the two studies raised some interesting questions.

"In a way the law change is putting the cart before the horse. Having identifiable donors will only work if couples tell their children.

"And the reluctance of couples from poorer backgrounds to explain the situation suggests education is an issue. We must work to de-stigmatise it."

Clare Brown, chief executive of support group Infertility Network UK, said if attitudes to donor treatment changed people would become more open.