Marriage Allowance was introduced by HM Treasury in April 2015.
Applying with Marriage Allowance Claims will allow you to automatically backdate your Marriage Allowance claim to include any tax year since April 2016 for which you are eligible for Marriage Allowance. Your eligibility for Marriage Allowance is subject to review by HM Revenue & Customs.
If your partner has passed away since April 2016, HM Revenue & Customs’ guidance states that you will generally still still be able to apply for Marriage Allowance, provided the eligibility criteria were met at the time. If your partner was the lower earner of the two of you, then person responsible for managing their tax affairs will need to apply to directly to HM Revenue & Customs.