The Institute for Fiscal Studies (IFS) issued a highly critical analysis of the move to strip the payments from some higher earners from next week.
While it made sense to target universal benefits paid to the better-off as part of efforts to tackle the budget deficit, the way it was being done was “problematic”, it said.
The IFS warned that some families would end up losing as much as 65p of every extra pound they earned – making it more likely they would work less or put more into pension funds to avoid the hit.
Households where one parent earns more than £60,000 a year will have to return the entire amount through the self-assessment system unless they have opted out of receiving it in the first place.
It will be taken away on a sliding scale where mothers or fathers earn between £50,000 and £60,000 – causing a significant rise in marginal tax rates for those families.
In its short report, the IFS said it expected the average loss to be £1,300 a year, with 820,000 families losing all state help and 320,000 having it cut.
It said the marginal tax rate faced by those with an earner of between £50,000 and £60,000 would rise by 11 percentage points for those with one child and another seven for each additional child.