Russian hackers have issued a chilling warning to tens of millions of Britons that they will wake up to "black screens and empty bank accounts".

Pro-Russia Unit 21 hacking group said they will hit the UK tonight at 9pm. "This is not a warning. This is a declaration," the group also said.

Their attack will be "anonymous" and "untraceable," Unit 21 also said in its message, adding it will come because the UK supports Israel in the war on Gaza. The group also claimed they have been "inside your networks [the UK's] for six months."

The full message from the pro-Russian hacking group read: "We are Unit 21. Pro-Russian hackers. From the darknet. For Russia. This is not a warning. This is a declaration. United Kingdom. You fund Zionists. You arm terrorists in Israel. You call it defense.

"We call it blood business. Your support for genocide in Gaza makes you a legitimate target. Your cyber infrastructure — government servers, power grids, financial systems — is now a war zone. We have been inside your networks for six months.

"BARON mapped every weakness. We know your emergency protocols. Your backup frequencies. Your ministers' private communications. Tonight at 9pm — first strike. Anonymous. Untraceable. Your citizens will wake up to black screens and empty bank accounts. Not dead. Paralyzed.

"Fear is better than death. Fear makes you question your leaders. You chose this war. We choose the battlefield. Stop funding terror. Or we shut you down piece by piece. No negotiation. No mercy. Unit 21. Watch the dark."

Other reports said the UK was also targeted recently, with Russian hackers hijacking Wi-Fi systems.

Last week, Radio Free Europe/Radio Liberty (RFERL) reported on a group of hackers connected to Russia’s military intelligence agency that snuck into thousands of internet routers used in homes and offices worldwide and secretly monitored e-mail traffic and website log-ons and passwords for months.

Russian hackers “compromised routers in the US and around the world, hijacking them to conduct espionage,” Brett Leatherman, assistant director of the FBI’s cyber division, said in a statement quoted by RFERL.

The group was also behind other attacks, the same media outlet said, like the one hitting the Democratic Party computers during the 2016 US presidential election campaign. US authorities later charged 12 Russians they identified as GRU officers with the intrusions.