Error messages appear when trying to complete an orderCredit: ROYAL MAIL
Others say they have been unable to make online payments and generate labels.
The "annoying" problems appear to have started at around 2pm, with 286 reports by 4.19pm.
Twitter is also flooded with people complaining about outages.
One man called Scotty said: "Is the Royal Mail website down again?
"Trying to pay for click and collect but keep getting errors."
Another user, Bonnie, posted: "Your website is not allowing [me] to purchase postage.
"I need to to pay for collection for tomorrow. Please fix."
A message on Royal Mail's website reads: "Something went wrong, please try refreshing your page."
Another alert shared by a customer says: "Sorry, our system is currently unable to process postage purchases.
"Our team is currently working to get things back up.
"Please try your purchase later."
A Royal Mail spokesperson said: “This was a temporary matter caused by planned site maintenance over the weekend that briefly overran.
"It is now fully resolved.
"We apologise for any inconvenience.”
