The main three web sites for the Royal Mail are currently unavailable, due to unknown technical problems.
Royalmail.com, parcelforce.com and postoffice.co.uk are all thought to have been inaccessible since last night. A Royal Mail spokesman would not comment on the precise cause of the problems, but said that "infrastructure issues" were thought to be responsible.
It is not known when operations are likely to resume. Parcel tracking services are still available to the public via telephone.