Under the marriage tax allowances, announced last month, couples will able to benefit from a tax break of up to £200 a year.
Mr Cameron claimed today that all basic rate tax-payers would benefit from the move.
But when shadow chancellor Ed Balls pulled him up on the claim in a point of order after Prime Minister's Questions, Mr Cameron was forced to admit the tax break would only be "available" to married basic rate tax-payers, where one half of the couple had an unused personal allowance which could be switched to their wife or husband.
In an answer to a question from Labour MP Tom Harris, Mr Cameron said: "What I can confirm is that all married couple paying basic rate tax will benefit from this move.
"I will tell you why I think this is important - it is not about the money, it is about the message. I think marriage is a great institution and I think we should be supporting marriage through the income tax system."
"Let me make this challenge: in government Labour gave a marriage tax break through the inheritance tax system. They gave a married tax break to the rich. I want to give a married tax break to everybody."
But Mr Balls said the Prime Minister was wrong to claim the move would benefit all married couples paying the basic rate of tax.
Raising a point of order, Mr Balls said: "Can I go back to the Prime Minister's answer to the Leader of the Opposition (Ed Miliband) on the marriage tax break?
"When asked whether it was true that a third of married couples would benefit, the Prime Minister said that all married couple that are basic rate taxpayers would benefit.
"Would the Prime Minister like to correct the record, because that is just not true?"
After he was i nvited to respond by Speaker John Bercow, Mr Cameron changed his wording slightly and said it would only be "available" to married couples paying the basic rate of tax.
He said: "The point is the married couples allowance is available to every basic rate taxpayer.
"I think that is something to celebrate in our country. I stand up for marriage, even if you want to talk it down.
"But can I thank you once again for your tenacity, even though you've been proved wrong on every major economic question that you're still in your place.
"You're the great election winner for us."
The tax breaks for married couples worth up to £200 a year will be introduced from 2015.
Under the scheme, married couples - where one half earns less than £42,285 and the other earns less than the personal allowance threshold - will be able to switch up to £1,000 from that unused personal allowance to their other half that pays the basic rate of tax.