Britain could launch military retaliation such as air strikes against a future cyber attack, the Defence Secretary has suggested.
Sir Michael Fallon warned potential attackers that a strike on UK systems “could invite a response from any domain – air, land, sea or cyberspace”.
The Defence Secretary said the UK’s ability to carry out its own cyber attacks against Islamic State in Iraq and Levant (Isil), also known as Daesh, had saved lives during the battle for Mosul in Iraq and the capability was also being used in the fight for Raqqa in Syria.
Meanwhile the head of the US Army said governments are relying too much on overstretched elite special forces such as the SAS and Delta Force to try to win conflicts.
Gen Mark Milley said it was a myth that special forces “can do it all” and they were being asked to conduct missions they were not designed for.
He spoke at a conference in London as the head of the Chief of the General Staff said the British Army was now at its smallest since the time of Oliver Cromwell.
British and American special forces have spent more than 15 years engaged in high intensity operations around the world since the 9/11 attacks.
The deployment of special forces became a favoured military tactic of Barack Obama and then Donald Trump as the US presidents tried to minimise their overseas military footprint.
Britain has also used special forces extensively in Iraq, Afghanistan, Libya and Syria in recent years.
Gen Milley, Chief of Staff of the US Army, told the Royal United Services Institute’s land warfare conference: “I’m a proud special forces soldier. Special forces are great, but they are special for a reason.
“They have special missions, special training and they are usually used for a special purpose and that purpose is not to win wars.
“Green Berets, SAS, SBS, Delta Force, Seal Team 6 – All the special forces in the world do not fight and win wars. Neither do armies, or air forces, or navies. Nations fight and win wars. They go to war as a nation and our forces … combined armed operations and joint operations win wars.
“There is a great tendency to place over reliance on and over stretch our special forces and ask of them things that they are not designed or capable of doing.”
Gen Milley also said it was a myth that wars could be won quickly or that armies could be built up quickly.
Speaking at the same conference, Gen Sir Nick Carter, the head of the British Army said reductions in troops meant generals needed to find ways to make up for a loss of numbers.
The Coalition government cut the regular army from 102,000 to 82,000 after the cost-cutting 2010 defence review.
Yet a bungled outsourced recruitment system, poor morale after cuts and a lack of operations have all combined to see numbers dip further to around 78,000.
Sir Nick said: “Mass is still important, but at what scale and how in our cases do we mitigate a lack of it?
“The British Army has not been smaller in regular terms since the days of Cromwell.”
Gen Milley, whose own army has been cut by around 100,000 since the height of the Iraq and Afghan wars, said it was a myth that armies were quick to generate.
He said: “There was a time when armies were easy to generate. That time has long gone. Land forces, land power is a very complex exercise in human interaction with the use of tanks.
It takes a long time to build those forces. It takes 15-16 years to build an F-16 pilot. It takes 15-16 years to build a platoon sergeant
“Armies are not quickly generated. Significant sized land forces are required if you want to achieve your political objectives in today’s world.”