SAS legend Chris Ryan said Britain needs to build its army up 'fast' - and hinted he'd be prepared to sign back up and fight, should World War Three erupt.

In a chilling warning, the famed special forces hero said the globe was at risk of being 'sucked into a vacuum' as war in Ukraine continues to spiral out control. 

The former military hardman fears the UK's army, which is the smallest it has been in more than 200 years, had shrunk so much it was at risk of becoming 'ineffective'.

His comments come as analysts warned Vladimir Putin had amassed more than half a million troops on the front line in a dramatic bid to overpower Ukrainian forces.

Speaking out, retired operator Chris said the Russian tyrant's latest effort to bring death and destruction to Ukraine must serve as a 'big warning' and that Western powers cannot afford to be 'caught on the back foot'. 

'This should be a warning to the world that if it goes down that route we could all be involved and sucked into a vacuum very quickly,' he told MailOnline. 

Ex-SAS Sergeant Chris Ryan has hinted that he would slip back into his military gear if World War Three erupted in his lifetime

Soldiers in Ukraine fire mortars from a frontline position in the war against Russia

A British-made tank is driven by a Ukrainian crew through the Donetsk region of Ukraine in March. Ryan has suggested Brits could end up backing Ukrainians on the ground

Ryan spoke to MailOnline ahead of the release of his new book, Traitor, which is inspired by real-world actions in the war against Russia

The former soldier spent a decade in Britain's elite military fighting unit before picking up a pen and writing both fiction and non-fiction books

'Our army has shrunk so much we’re nearly becoming ineffective... We don’t have the manpower. We need a professional army and we need it fast.'

And asked if he would consider slipping back into his military kit, should war spread, Chris added: 'At my age I would be part of a "Dad’s Army" crew but if it goes to hell, we will all be doing it.'

The warning came as Russia continued its advances in northern Ukraine, which threaten the country's second city, Kharkiv.

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Putin's marauders have already captured several villages on the eastern border, which Ukraine had previously liberated during the opening salvo of the war. 

The advances, feared to be part of Moscow's major summer offensive, 'caught the West napping', Defence Secretary Grant Shapps hinted this week.

In a plea to the Government, Chris - who spent a decade in Britain's elite military unit fighting around the world before becoming a best-selling author - urged action.

He feared the war was entering a deadly new phase which would see 'missiles being thrown around' that could strike Nato countries like Poland, Germany, France and the UK - plunging the world into all-out war. 

It comes as Russia threatened to strike British military installations and ordered its troops to hold battlefield nuclear weapon drills, in a disturbing ramping up of rhetoric from the Kremlin.

The sabre rattling was in response to French president Emmanuel Macron's comments on western troops fighting in Ukraine and British foreign secretary, David Cameron, on using British-supplied weapons against Russia.

'We should prepare ourselves because history has a way of repeating itself,' added Chris. 'We should be bolstering our munitions factories because our stockpiles have been depleted.' 

The SAS veteran's comments come as he prepares to release his latest blockbuster thriller, Traitor.

Set in the middle of the Ukraine war, the novel follows Sergeant Major Luke Carter, a decorated hero of 22 SAS, who is sent to Australia on a secretive recruitment job.

The elite soldier and his team of Australian SAS troopers are then thrust into a high-stakes operation to take down key Russian targets and change the course of the conflict. 

But unbeknown to them, their efforts are being sabotaged by Russian spies lurking in Ukraine, whose underhand tactics threaten to lead to the war spilling out across Europe - and beyond. 

Russia's invasion of Ukraine, which began more than two years ago, has devastated the country (pictured: rescuers tackle fires following a missile strike on Kharkiv on May 10)

Russia's ongoing attacks in the country, including against civilian targets, have left much of the country battle scarred (pictured: residential buildings in Kherson struck by Russian fire)

The war in Ukraine shows no signs of abating while Russian president Vladimir Putin (pictured) continues his campaign in the country

Speaking to MailOnline, Chris - a former SAS Sergeant who famously evaded capture in the Gulf War after a mission behind enemy lines in Iraq went catastrophically wrong - said the inspiration for his book was based on real-world action.

'A year ago, a group of anti-Kremlin saboteurs hit the headlines after they crossed into Russia from Ukraine and launched a series of raids across the Belgorod region,' he said. 

'Recruited from the ranks of army defectors and immigrants living in Ukraine, their stated aim is to topple Vladimir Putin’s regime. 

'Although these groups would appear extremely unlikely to succeed, as I watched the news reports the seed of an idea swiftly took shape in my head. What if these militia forces were trained by the SAS? 

'Or – even better – what if the "Regiment" found themselves in a position that meant they had to carry out the attacks? After all, operating behind enemy lines has been the trademark of SAS operations for decades, going back to the desert raids in the Second World War and, more recently, the attack on Pebble Island in the Falklands campaign. 

'The prospect of British soldiers on the ground in Russia might seem outlandish. But as we’ve seen in the past, wars can escalate quickly. With the conflict in Ukraine dragging on, and no end in sight, there’s always the risk that one side might miscalculate, or push the envelope too far. And if that happens, things could get out of hand very fast – with potentially devastating consequences.'

• Traitor hits the shelves on May 23 and is available for pre-order from all good bookshops and on Amazon, now.