The former deputy supreme commander of NATO argued it is time to “think the unthinkable” and consider bringing back conscription to deter Russia from igniting World War 3 with the Western military alliance.

The head of the UK Army, General Sir Patrick Sanders, said on Wednesday that Britain should lay the foundations for a voluntary call-up if a war broke out and train a “citizen army” ready to fight a potential conflict on land.

But General Sir Richard Shirreff, who was a NATO chief between 2011 and 2014, believes it unlikely the UK government would be able to persuade a large number of civilians to sign up voluntarily given army recruitment is in freefall.

In a letter to The Times, General Sir Richard argued it is now time to look at conscription in order to beef up the British army.

He wrote: “Although the reserve army can go some way to reinforcing the regulars, now is the time to think the unthinkable and look carefully at conscription.

“To most professional soldiers (myself included), conscription is anathema. However, if deterrence is to be effective, Russia deterred and catastrophe averted, it might be necessary.”

In his letter, General Sir Richard also questioned whether the Ministry of Defence (MoD) is prepared for a war of a “scale and ferocity that would engulf the UK” if NATO had to leap at the defence of a fellow member under attack in accordance to its principle of collective defence.

General Sir Patrick’s warning, issued during a speech at an armoured vehicle conference, sent shockwaves through the country, sparking a widespread discussion on what a mass mobilisation would mean to everyday Britons.

The outgoing Chief of the General Staff added: “We will not be immune and as the pre-war generation we must similarly prepare – and that is a whole-of-nation undertaking. Ukraine brutally illustrates that regular armies start wars; citizen armies win them.

Downing Street and the MoD commented on Wednesday that there was “absolutely no suggestion of a return to conscription”.

UK Prime Minister Rishi Sunak‘s spokesman also said hypothetical scenarios of a future potential conflict were not helpful.

UK defence leaders in the MoD are reportedly discussing how to build up a force of up to 500,000 troops and civilians ready to take on a nation such as Russia, which has fully adopted a war economy following its invasion of Ukraine.

This comes as other European nations have warned their citizens they need to mentally prepare for the possibility of a war sparked by Russia attacking NATO’s eastern flank.

Sweden was the first to raise alarm bells, followed by Germany and Norway.