Malcolm and Pauline Dixon are hoping to adopt a Cambodian child

The ban followed concern that measures were not in place to prevent adoptions without birth parents' proper consent.

But Helen Mountfield, representing the couples, said it was unlawful to halt adoption procedures already under way.

The couples, from Cardiff, London, Somerset, Sussex, Hertfordshire, and one unnamed couple, argue there could have been measures short of a ban.

Human rights claim

The High Court heard the government's action, also taken in other Western countries, followed growing corruption and child trafficking in Cambodia.

Ms Mountfield said: "The claimants don't dispute the existence of a very great social evil in Cambodia of child trafficking.

"Nobody before the court seeks to argue the problems does not exist, or seeks to suggest that anything other than the best interests of the children and the elimination of child trafficking should be at the centre of decision-making."

Cambodia is in the grips of a major humanitarian crisis

She said she was not arguing for the right for British couples to be allowed to "adopt a child at any price".

But she said that in the case of the unidentified couple who had adopted one Cambodian child and wished to adopt a second, the ban meant their two-year-old was denied a sibling from her own ethnic and cultural background.

She said this breached the child's rights to a private and family life under the European Convention on Human Rights.

That child is bringing her own challenge for the right to have such a sibling.

Aids epidemic

And the couple from Taffswell, Cardiff, Peter and Lesley Maksimovic, were NHS workers who had spent thousands on seeking the right to adopt.

Mr Justice Munby heard the case took place in the context of "a major humanitarian crisis in Cambodia" after the Aids epidemic.

Statistics for 2003 showed there were approximately 670,000 orphans under 18 in Cambodia - 5% of the population.

Out of those, 30,000 were Aids orphans under the age of 15.

The other couples involved in the action are Hugo and Caroline Bottellier, of Challoner Crescent, West Kensington, London; Dr Simon Capp and Jan Walters, of Higher Backway, Bruton, Somerset; Malcolm and Pauline Dixon, of High Cross, Rotherfield, East Sussex; and Eivind and Fiona Dullforce, of Newberris Avenue, Radlett, Hertfordshire.