Around 1.1 million families will be affected by changes to the previously universal benefit that come into force on Monday.
But a panicked HM Revenue and Customs had only written to 800,000 families, blaming incomplete data on some people following changes to their income, relationship status or address.
HMRC said it was employing "extensive" media activity to try and get the message across.
But Labour criticised the gap of 300,000 people who have not been informed, saying parents could be in for a "nasty surprise."
Labour MP Catherine McKinnell added: "This policy not only risks being a costly administrative nightmare, it's also unfair."
From next week, families in which one parent earns more than £60,000 a year will lose all their benefit, which is currently £20.30 a week for the first child and £13.40 for each child after that.
Families where one parent earns between £50,000 and £60,000 will have the benefit reduced on a sliding scale.