The US and the UK are to mount cyber attacks on one another in a joint "war game" to test each country's ability to combat online crime.
David Cameron announced the "unprecedented" arrangement during talks with his US counterpart, Barack Obama.
The first of a series of games will take place later in 2015 and will involve an attack on the Bank of England and commercial banks, targeting both Wall Street and the City of London, the BBC reports.
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The test will be followed by a range of "further exercises to test critical national infrastructure", Downing Street said in a statement.
The two leaders also announced the formation of new "cyber cells" comprised of agents from MI5 and the FBI, who will collaborate to improve the way the UK and US share information about threats. The cells represent the first the UK has established with another country.
The new measures come in the wake of the devastating cyber attack on Sony Pictures by a group calling itself the Guardians of Peace. The FBI blamed North Korea for the hack, which led to the early release of a number of films and publication of embarrassing private correspondence and personal data.
Cameron told the BBC that cyber attacks are "one of the big modern threats that we face".
He said the UK is already well prepared to defend itself against cyber attacks, but that more can still be done.
"This is a real signal it's time to step up the efforts and to do more," he said. "It's also about protecting people's data, people's finances – these attacks can have real consequences to people's prosperity."
On Monday, the US military's Central Command Twitter account was attacked by a group claiming affiliation with Islamic State.Obama said that the hack, though relatively unsophisticated, underlined the "urgent and growing danger" of online crime.
