Technology has made it easier for terrorists to mount attacks, the Attorney General will warn as he sets out the legal basis for British military strikes against targets overseas for the first time.
Jeremy Wright QC will stress that it is vital for the UK to have the right to deploy lethal force in self-defence, arguing that the law must keep up with "changing times".
He will say the frontline has "irretrievably altered" with technology used to evade law enforcement and inspire attacks around the world that previously would have taken months of planning.
The Government's top law officer will use a speech in central London to provide an unprecedented insight into the considerations that must be weighed up before action is taken against extremists abroad.
Scrutiny of the legal basis for such operations intensified in September 2015 when then prime minister David Cameron revealed that an RAF drone had killed two British jihadists in Syria the month before.
Reyaad Khan, 21, from Cardiff, was the target of the attack launched near the Islamic State stronghold of Raqqa, while a second UK national, Ruhul Amin, also died.
Last year MPs and peers called for ministers to urgently clarify the legal basis for launching drone strokes against IS targets.
In his speech on Wednesday night Mr Wright will set out, in greater detail than has been disclosed before, the legal considerations that would be discussed before action is taken in self-defence against an imminent attack.
Under the approach lethal action will always be a last resort, when there is no other option to defend the country from attack and no other means to detain, disrupt or otherwise prevent those plotting acts of terror - for example, law enforcement measures.
A state may use force in self-defence not only in response to armed attacks but also in certain circumstances, to prevent attacks before they occur.
Mr Wright will say: "The UK is a world leader in promoting, defending and shaping international law – and for the first time we are setting out how we determine whether an attack is imminent.
"We are a long way from being able to see troops massing on the horizon. The frontline has irretrievably altered.
"Technology has made it easier for terrorists to carry out attacks.
"The law has to keep up with the changing times.
"The Government has a primary duty to protect the lives of its citizens. But it can only use lethal force where there is a clear legal basis for doing so."
Mr Wright will say any government determining whether to take action must consider how certain it is that an attack will come, how soon it will be and on what scale.
He will add that it must then be determined whether anything could credibly be done to prevent the attack, and whether it is the last clear opportunity for preventative action.
For more than two years the official threat level for international terrorism in the UK has stood at severe, meaning an attack is "highly likely".
Britain's security services and counter-terrorism units have foiled at least 10 attacks in the past two years.