Twins Siôned and Eleri are entitled to only one baby tax payment
A mid Wales MP has described as an "outrage" baby tax credit payments which he says penalise the parents of twins or triplets.
Montgomeryshire MP Lembit Opik has taken up the campaign for payments for both babies on behalf of his constituent Gary Northeast, the father of one-year-old twin girls.
Under the new child tax arrangements, parents get an additional payment - but only one regardless of how many babies they have within one year.
Mr Opik says that it is a form of punishment for couples who "by an oversight of birth" happen to have twins or triplets.
"The only other country that I can think of who have punished people for having more than one child was China in the 1950s and 1960s," said the MP.
"I sincerely hope that...the government is not providing a disincentive to twins."
"It really is an outrage that something which is the hand of biology or fate can lead to over £500 a year difference to parents in that situation.
Musician Mr Northeast and his wife Marilyn Hunt, a primary school teacher, from Dolanog near Welshpool, only received the one payment of £545 for their twins - Siôned and Eleri.
They contacted their MP after becoming involved in a campaign by the Twins and Multiple Birth Association (Tamba).
"The baby element of the child tax credit is only paid for one child, so if you've got twins you are losing out to the sum of £545, " said Mr Northeast.
From April the New Tax Credit replaced the old benefits and tax credits with a single system.
All families with an income up to £66,000 a year are eligible to claim the baby tax credit if they have a child under a year old.
Mr Opik and other Liberal Democrats MPs have signed an early day motion calling on the government to recognise the discrepancy.
But the minister responsible for the new child tax credits - Paymaster General Dawn Primarolo - has said the payment in question was to recognise the responsibility taken on by all families with children.
She added that it was not intended to reflect the number of children in a family as the individual child payment already did that.