Currently, there are various ways for Britons to claim Pension Credit if they think they could be eligible.
Applications can be submitted online, via phone or by post.
However, Ms Morrissey warned Christmas post backlogs may impact the speed at which a claim is possible via post.
As a result, she encouraged people to explore the other options to ensure their claim reaches the DWP as soon as possible.
She added: “If you think either yourself or a loved one might qualify then it’s well worth picking up the phone or getting online before the December 18 deadline to see if you can make a claim.
“There have been reports that DWP are dealing with a backlog after a surge in applications so there may be some delay in processing your claim but if you are eligible and you submit your claim in time you will benefit.”
