It cannot be right for children to be sold to the highest bidder

William Hague

But a Californian couple who also claim custody of the girls, have promised to do everything in their power to get them back.

Mr Kilshaw, a 45-year-old solicitor, and his wife, 47, planned the adoption on the internet and brought the girls to Britain on six-month tourist visas.

They say they intend to apply for British citizenship for them, but could now face a long legal battle over the twins.

The couple's local authority, Flintshire County Council, says it is now monitoring the welfare of the twins and has requested to see copies of the US adoption documentation.

Adoption law reform

Mr Blair told the Commons on Wednesday: "I think everyone feels it is absolutely deplorable that children are traded in this way."

The government published a White Paper on adoption before Christmas and intended to introduce subsequent legislation during the current parliamentary session, he said.

It was important to clear away some of the "bureaucratic obstacles" to adoption, as well as to prohibit such "trading" of children, Mr Blair added.

Tony Blair: Determined to act

The prime minister was responding to Tory leader William Hague who pledged opposition support for reform of adoption law.

Mr Hague said: "It cannot be right for children to be sold to the highest bidder."

Social workers visited the Kilshaws on Wednesday afternoon after which the director of education and children's services at Flintshire County Council, Keith McDonogh, issued a statement.

In it, he said the council would be "monitoring the well-being" of the children.

"We have duties and responsibilities as a local authority for the welfare of these children," he said.

"The nature of those duties and our powers depends upon the status of the adoption.

"We are awating clarification from the Home Office, the Foreign Office and the National Assembly on the international implications of this case and the status of the adoption."

Parents hit back

Mrs Kilshaw responded to the prime minister's description of the adoption as "unacceptable".

She told ITV News: "We have not broken any American laws and as far as I'm concerned we haven't broken any British laws, so what is unacceptable?"

The Kilshaws, who already have two children together - James, seven, and Rupert, four - as well as Cayley, 18, from Mrs Kilshaw's previous relationship - looked to adopt because they wanted another daughter.

Californians Richard and Vickie Allen say they paid a US baby broker to adopt the twins themselves and would fight to get them back.

'A lot of prayer'

Mr Allen said: "It's going to take a lot of co-ordination, money and effort on everyone's part and a lot of prayer."

"Hopefully God's on our side and he's going to let the girls come home to us."

FBI spokesman Matthew McLaughlin in Los Angeles confirmed on Wednesday that the bureau had launched a preliminary investigation into the case.

Tranda Wecker, the children's natural mother

The babies were given up for adoption by their natural mother, Tranda Wecker, 28, from St Louis, Missouri.

Mr and Mrs Kilshaw have denied knowing about a deliberate attempt by Ms Wecker to defraud the US authorities in order to get the girls adopted.

They said it was "totally untrue" that they had known the girls' mother had lied about her address in order to have the children adopted.

'Stable home'

The case has sparked widespread condemnation from family groups in the UK.

Felicity Collier, chief executive of the British Agencies for Adoption and Fostering, said the twins needed a stable home.

"In our view they want to be with a family who are approved of as suitable and it may well be that this family is going to provide a loving and caring home," she said.

"But the manner in which they were brought into the country and the exchange of money, clearly gives rise to great concern as does the apparent bidding of the adopters."