As part of a new joint defence against online criminals, the UK and US are to carry out war game cyber attacks on each other.
The first exercise will be a staged attack on the financial sector and will take place later this year.
Agents are said to co-operate in "cyber cells" on both sides of the Atlantic.
The arrangement between the two countries was announced as Prime Minister David Cameron held talks with US President Barack Obama.
Downing Street said the "cells" involving M15 and the FBI were the first the UK had established with another country.
Their aim is to improve the flow of information about threats between the US and the UK.
The measures come in the wake of the recent hacking of Sony Pictures' computers and the US military's Central Command's Twitter feed, where comments were posted promoting Islamic State (IS) militants.
Political editor Nick Robinson 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."
It has also been announced that money will be available to train the next generation of cyber agents.
In 2013, The UK's National Audit Office warned in that a lack of skilled workers was hampering the fight against cyber crime.
David Cameron said the UK was already prepared for a cyber attack, saying that GCHQ had "massive expertise" but still felt however that more needed to be done.~
He said: "This is a real signal it's time to step up the efforts and to do more.
"It's also about protecting people's data, people's finances - these attacks can have real consequences to people's prosperity."
Return to internet news headlines
View Internet News Archive