By Daniel Hunter
The UK and US will carry out cyber attacks against each other in a "war game" to improve cyber security.
It is the flagship move in a new joint effort to defend against online criminals.
The first of the 'war games' will take place later this year, with the financial sector the first to be targeted, Downing Street said.
It comes after talks between Prime Minister David Cameron and President Barack Obama in Washington yesterday (Thursday).
Agents from MI5 and the FBI will also co-operate in "cyber cells" in both countries. It is the first time the UK has deployed such cells with another country.
The partnership comes after a string of high profile cyber attacks, particularly on the US. In the past month, Sony has been the subject of several attacks, including one that threatened the public and resulted in its movie business delaying the release of a film about a plot to kill North Korea leader Kim Jong Un, The Interview.
The US Central Command Twitter and YouTube accounts were also attacked by hackers claiming to support Islamic State.
In 2013, the National Audit Office said a lack of skilled workers was affecting the fight against cyber crime.
"This is a real signal it's time to step up the efforts and to do more," the Prime Minister said.
"It's also about protecting people's data, people's finances - these attacks can have real consequences to people's prosperity."
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