The Royal Mail said it is planning to carry out additional deliveries this Sunday as the severe weather that has led to a backlog of post is expected to return.
Up to 7,000 delivery rounds - serving more than one million homes and addresses - are planned.
Mark Higson, managing director of Royal Mail, said: 'This is already the worst December weather the UK has seen for almost 30 years.
'We are planning up 7,000 delivery rounds this Sunday as we continue our drive to deliver items as quickly as possible.
'We will continue to do everything in our power to deliver as quickly as possible.'
The Royal Mail is also asking customers to help postmen and women deliver in time for Christmas by placing orders and posting gifts and cards as soon as possible.
Its latest recommended posting dates for pre-Christmas delivery are:
this Saturday, for Second Class items;
Tuesday for First Class mail and packets;
and Thursday, December 23 for special delivery.
For more information on deliveries go to royalmail.com/serviceupdates