Apply for an increase in tax credits for a backdated disability benefit decision.
If you have received a backdated disability benefit decision covering a period when you were claiming Working Tax Credit or Child Tax Credit, you may be eligible for an increase in your tax credits. You must submit your application within one month of being notified that you qualify for the benefit or that your benefit rate has increased. To apply, you must complete the relevant TC-DB1 or TC-DB2 form and email it to HMRC.
Source: https://www.gov.uk/guidance/apply-for-an-increase-in-tax-credits-for-a-backdated-disability-benefit-decision. Summarised automatically from the official source — general information, not advice.