Deliveries have been suspended with immediate effect.
A spokesman said: “Our postmen and postwomen are working very hard across the UK in challenging conditions. As we said at the start of the coronavirus crisis, there will be some disruption to services.
“We understand the importance of the postal service in keeping the UK connected at this time. We have also listened to our hard-working colleagues who have asked us to ease the additional burden on them if possible.
“We will temporarily no longer deliver letters on a Saturday. We will continue to provide a letter delivery service from Monday to Friday and continue to deliver Special Delivery, Tracked, all non-account services and most other parcels from Monday to Saturday.”
Letters can still be posted on Saturdays and collections from businesses, post offices and post boxes will take place as normal.