A ransomware cyber attack has struck some of the world's biggest companies, the National Cyber Security Centre has confirmed.

The National Cyber Security Centre, which is part of GCHQ, said: "We are aware of a global ransomware incident and are monitoring the situation closely.

"The NCSC website provides advice to the public and business on how to protect your digital systems."

International advertising firm WPP said a number of its companies have been affected and it is currently "assessing the situation".

While the FTSE 100 company gave no more detail about how the business had been impacted, its website appeared to be down as it made the announcement.

In a statement, WPP said: "IT systems in several WPP companies have been affected by a suspected cyber attack.

"We are assessing the situation, taking appropriate measures and will update as soon as possible."

Global law firm DLA Piper, which has offices in London and other parts of the UK, also confirmed it had been affected.

A spokeswoman said: "We are taking steps to remedy the issue as quickly as possible."

It is understood the company has taken its email system down as a preventative measure.

It came as company and government officials reported major disruption to the Ukrainian power grid, banks and government offices.

Russia's Rosneft energy company also reported falling victim to hacking, as did shipping company AP Moller-Maersk, which said every branch of its business was affected.

Ukrainian deputy prime minister Pavlo Rozenko posted a picture of a darkened computer screen on Twitter, saying the computer system at the government's headquarters has been shut down.

There is very little information about who might be behind the Eastern European disruption, but technology experts who examined screenshots circulating on social media said it bears the hallmarks of ransomware.