Claiming certain tax credits can impact on other benefits received by a claimant. HMRC have provided guidance which outlines when and how benefits such as Council Tax Benefit, Housing Benefit, Income Support , income-based Jobseeker's Allowance, income-related Employment and Support Allowance and Pension Credit count tax credits when means testing entitlements to benefits.
Benefits such as Income Support income-based Jobseeker's Allowance income-related Employment and Support Allowance Pension Credit Housing Benefit and Council Tax Benefit may go down on claiming the Working Tax Credit and/or Child Tax Credit.
HMRC recommends contacting the benefit office here and ask them to check how tax credits could affect a particular benefit payment before a claim to tax credits is made.