You might be overpaid tax credits if
there is a change in your circumstances (external link) - even if you report the change on time
you or the Tax Credit Office make a mistake
The Tax Credit Office will write to tell you what you owe and how to repay the money. If you think they made a mistake, call the helpline (external link). If you still get tax credits or are now getting Universal Credit, the money you owe will usually be taken from your future payments.
If you no longer get tax credits and you do not get Universal Credit, you will have to pay HM Revenue and Customs (HMRC) directly (external link). The money may be recovered from you in another way if you do not repay HMRC in time.