The Liberal Democrats have accused the prime minister of creating a tax system which discriminates against millions of unmarried couples and their children.

Speaking at prime minister's questions on Wednesday, Lib Dem acting leader Vince Cable asked Gordon Brown if he believed there was a moral case for rewarding marriage through the tax system.

Brown said: "It is because we recognise marriage in the tax system that we have done and made the changes we have on inheritance tax."

On children's tax credits and child benefits, the prime minister said it was the "duty of every citizen of this country... to support not just some children in our country, but all children".

Cable, who is also the Lib Dem Treasury spokesman, attacked the prime minister for "lining up with the Tories to defend the principle that these families should not merely be condemned to the everlasting flames of hell, but should be taxed more on the way".

But arguing that the Lib Dems had not put forward detailed proposals on the issue in recent years, Brown said it "has always been the case that marriage is recognised in the inheritance tax system".

The prime minister pointed to £18bn worth of spending commitments from the Lib Dems, and warned that if the party's proposals were adopted on inheritance tax and extended it to everyone, "that would be a very great expense that is additional indeed".

"I do not know how the Liberal Party policies would be able to cope with yet another spending commitment," Brown said. "Because in the last few days we've had commitments to a border police force, high speed rail links, more money to Visit Britain, reducing VAT on historic buildings - £18bn of spending commitments in all.

"And the most recent one I want to draw attention to is more investment in bullying prevention. Perhaps they should look at it as a party."