LONDON (Reuters) - The head of Britain's national sperm bank urged men to prove their manhood and help ease a shortage after the center signed up just nine registered donors in the year since its creation.

"If I advertised saying 'Men, prove your worth, show me how good you are', then I would get hundreds of donors," Laura Witjens, the chief executive, told the Guardian newspaper.

"That's the way the Danish do it. They proudly say, this is the Viking invasion, exports from Denmark are beer, Lego and sperm. It's a source of pride."

Denmark's biggest sperm bank, privately-owned Cryos, said it had more than 450 registered donors and was exporting sperm to more than 80 countries.

The Guardian said the advertising campaign's "superman" theme had a serious message as donors had to have strong sperm for it to survive the freezing and thawing process.

The rigors of this process is one of the reasons the clinic has so few donors after almost a year, Witjens was quoted as saying.

She told the BBC that generally the bank got one registered donor from each 100 enquiries. An approved donor has to come to the clinic twice a week for up to four months and refrain from sex or masturbation for two days before each visit.

Donors receive 35 pounds ($54) per session from the National Sperm Bank, a joint project between the National Gamete Donation Trust and Birmingham Women's Hospital.

"You shouldn't do it for the money, there's really not much in it for that," she told the BBC. "We're literally asking men to come forward ... and to help people to become a family."

Human sperm is on sale from some private companies in Britain for as much as 950 pounds per sample. It is also available for purchase on the Internet.

British regulations on anonymity introduced in 2005 mean that all children conceived as a result of sperm donation have the right at the age of 18 to identifying information about the donor.

(Reporting by Guy Faulconbridge and Kate Holton; Additional reporting by Annabella Nielsen in Copenhagen; Editing by Raissa Kasolowsky)