Deliveries are expected to be affected because of the fire
About 150 staff have been evacuated after a fire broke out at the Royal Mail sorting office in Cardiff.
Police said the fire was contained in an area at the back of the Penarth Road building, well away from the road.
It is believed the fire started in a loading bay and some vans parked there were damaged. No-one was injured.
Royal Mail warned there will be some disruption to postal services. Customers wanting to collect mail or parcels are asked to stay away.
The fire broke out at about 0845 GMT on Monday.
Paul Kelly, Royal Mail general manager, said: "It started in a tray with equipment which was outside. There were some vans parked in that loading bay which we believe have been damaged."
All 150 staff at the depot were sent home for the rest of the day.
There will be local collections in Cardiff as normal but we will probably bring forward the time of the last collection
Royal Mail general manager Paul Kelly
Mr Kelly added: "There will be no deliveries in the residential areas today. Most of our business mail has already gone out.
"We'll be starting our contingency plan now."
He said they were in touch with their Swansea office and would be contacting colleagues at Bristol and Gloucester for assistance.
Mr Kelly added: "There will be local collections in Cardiff as normal but we will probably bring forward the time of the last collection."
Postman Chris Williams was on his way into work when he spotted a "huge plume of grey smoke" coming from the building.
The fire broke out at about 0845 GMT (Picture from postman Matt Henwood)
He said he and many others were in a shelter on the other side of the building from where the fire had broken out.
He said he could not see any flames, but had heard the fire engines arrive.
Another postman, Matt Henwood, said: "At the moment, nobody knows what started the fire.
"It was about 8.30am when we got thrown out of the building," he added.
Staff wait outside the Royal Mail building while the fire is dealt with
Royal Mail said customers in the CF10, CF11 and small parts of CF23 and CF24 postal areas will have their mail delivered as soon as post men are able to get into the building again.
Staff will stay on beyond their normal working hours to make sure any backlogs are cleared.
It is not yet known what has been damaged in the fire as staff cannot get in to assess the situation.
However, a spokesman said most of the mail would either already have been sent out to be delivered or is in a separate room.
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