I will receive my first work pension payment in Mar 2021 retroactive to Dec 2020. Do I declare the December 2020 part as 2020 income or do I pay tax on all in 2021?

In other words, will I be paying taxes on 13 months of work pension payments in 2021, since it all arrived in 2021, or am I expected to declare the December portion as 2020 pension income?