Reuters/Jonathan Ernst
Cyber attacks will be carried out by the countries' intelligence services, the MI5 and the FBI.
First up is the financial sector: The Bank of England and commercial banks in the City and Wall Street are going to be targeted to see how well businesses can cope with hacking dangers. Transport services will also be hit in the digital battles.
The move, announced today, will "test critical national infrastructure" and is in direct response to recent attacks on Sony Pictures and the US military's Central Command Twitter feed, the BBC explains.
UK prime minister David Cameron announced the project with US president Barack Obama during a two-day visit this week. As well as practicing attacks, the nations want to improve data flow to better determine what's coming in and out - and what information is safe.
Cameron specifically referenced the alleged attack by North Korea on Sony before the talks, AFP reports. Obama and Cameron met for dinner yesterday and were set to hold meetings on Friday.
The prime minister posted a video on his Twitter account and says: "I think it really matters that Britain and America properly cooperate and work together."
"We'll be making some important announcements about how we keep our businesses and our people safe from these new cyber threats," he adds.
Protecting our economic and financial security - a video clip with my thoughts ahead of a meeting with @BarackObama:
https://t.co/2UIPsrspxq
- David Cameron (@David_Cameron) January 15, 2015