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People sending items abroad with Royal Mail have been warned there is no end in sight to delivery disruption after the firm was hit by a "cyber incident" on Thursday.

The firm is still unable to send letters and parcels overseas and says it is "working hard" to fix the issue.

There are also minor delays to post coming into the UK, but domestic deliveries are unaffected.

Royal Mail apologised again and vowed to update people as soon as possible.

It said that some customers who had posted items abroad even before the "incident" might see delays.

The company is calling it a "cyber-incident" rather than a cyber attack because the problem is still under investigation, the ����ý has been told.

A National Crime Agency spokesperson said it was "aware of an incident impacting Royal Mail" and was working alongside the National Cyber Security Centre - which is part of the UK's cyber intelligence agency GCHQ - to understand its impact.

The back office system that has been affected is used by Royal Mail to prepare mail for despatch abroad, and to track and trace overseas items.

It is in use at six sites, including Royal Mail's huge Heathrow distribution centre in Slough, as well as its Bristol site.

In the year to March, Royal Mail sent 152 million parcels abroad which equates to about 200,000 items a day.

However, that was a small fraction of the number of parcels it sent domestically.

Andrew Pope runs an online business, PopeyMon Games, which sells vintage games and refurbished computers to customers in the UK and around the world.

Based in Southampton, Andrew says he was forced to look at an alternative courier for his UK deliveries before Christmas due to parcels being left on doorsteps and what he describes generally as a poor service.

He describes the cyber incident as "the latest nail in the coffin" and says he will now look at moving away from Royal Mail for his international sales, which make up about 20% of his income, and are generally high-value items.

He adds, as a former temporary postal worker himself: "I really despair... this latest incident is just another example of a race to the bottom."

Ciaran Martin, a professor at the University of Oxford and former chief of the National Cyber Security Centre, told the ����ý he believed the "incident" was down to "malicious activity".

He suggested, based on the limited information about the case so far, that it was likely to be criminal extortion or a "ransomware" attack.

"You're locked out of the system and there will be a demand, probably in broken English from criminal abroad, to pay a lot of money in cryptocurrency for what is called a decrypt key to let you back in to the system," he told the Today programme.

Whatever has happened, a full recovery is likely to take some time, Prof Martin added.

Royal Mail has faced a number of hurdles in recent months, including mounting losses as people send fewer letters and delivery delays as postal workers strike over pay and conditions.