Deputy Prime Minister Nick Clegg has said axing child benefit for the richest families was an "excruciatingly difficult" decision.
Letters will be going out from HM Revenue and Customs from Monday to advise around one million households on £50,000 or more how their payments will change.
Conservative backbenchers fear a backlash from traditional Tory voters over the move that will end the principle of universal entitlement to child benefit.
Lib Dem Mr Clegg said the principle behind "making these excruciatingly difficult decisions" was ensuring the burden falls "as fairly as possible".
"I don't expect them to welcome this, I perfectly understand why people who don't feel wealthy - who may not feel wealthy at all given all the costs they face today - don't like this change but I will ask them just to reflect for a minute there are many, many others, the vast majority of people in this country who are on much lower incomes than them, who are also having to make sacrifices."
A Downing Street spokeswoman said: "It's very difficult to justify continuing to pay child benefit to the wealthiest 15% of families."