The UK and US are to carry out "war game" cyber attacks on each other as part of a new joint defence against online criminals.
The first exercise, a staged attack on the financial sector, will take place later this year, Downing Street said.The "unprecedented" arrangement between the two countries was announced by Prime Minister David Cameron ahead of talks with US President Barack Obama.
The two men discussed a range of other issues, including counter-terrorism.
They are holding a press conference in the Oval Office of The White House after talks lasting about an hour.
He is expected to talk to the US president about getting companies such as Google and Facebook to allow governments to view encrypted messages.
'Modern threat' In terms of the planned cyber war games Downing Street said they will aim to improve the flow of information between the US and UK about threats.
No 10 said agents will co-operate in "cyber cells", involving MI5 and the FBI, and they will be the first the UK has established with another country.
Speaking to BBC political editor Nick Robinson after arriving in Washington on Thursday night for a two-day visit, Mr Cameron said cyber attacks were "one of the big modern threats that we face".
The first war game will involve the Bank of England and commercial banks, targeting the City of London and Wall Street, and will be followed by "further exercises to test critical national infrastructure", Downing Street said.
Money will also be made available to train "the next generation" of cyber agents.
The tensions and confusions over what cyber security means are all too apparent this week.
Is it about defending corporate networks against hostile attackers of the type who targeted Sony? That's the focus of today's announcements about war-gaming and threat cells.
Or is it about getting hold of data and communications about terrorists? That seemed to be the focus earlier in the week, with briefings that the visit would focus on getting US companies to be more helpful in providing data to British authorities.
The two are different in focus and it is not yet clear how much progress on the latter the prime minister will make with a president whose relations with the tech sector are already difficult post-Snowden.
There is also some tricky overlap between the two fields, for instance on how far information should be encrypted so it cannot be read or stolen.
Encryption may foil foreign cyber spies but also stymie law enforcement.
The cyber attack on Sony Pictures led to data being leaked from its computers exposing emails and personal details about staff and stars.
The hackers, who called themselves #GOP or Guardians of Peace, also threatened cinema chains planning to screen Sony's satirical North Korea comedy, The Interview, the plot of which involves a bid to assassinate the country's leader Kim Jong-un.
Sony initially cancelled the film's release after leading US cinema groups said they would not screen it, a move which Mr Obama later described as "a mistake"
