BRITAIN could soon be at war with Russia, a retired Army officer has warned.

General Sir Richard Shirreff was speaking to BBC Radio 4’s Today programme when he said any incursion into Nato territory would bring Britain into direct involvement in the conflict.

“There is no surprise and there is profound sadness and a sense of appalling horror at what is about to unfold for the people of Ukraine,” he said.

“And I think we have to assume that this is not Russia biting off a chunk of Ukraine – for example, establishing a land corridor in Crimea – but a full-blown military offensive to occupy Ukraine.”

Sir Richard said it is “entirely plausible” that Russian President Vladimir Putin could be aiming to revive the Soviet Union, adding that if Russia puts “one bootstep” into Nato territory, the entire alliance will be at war.

Asked whether Britain might be expected to take part directly in the current military confrontation, he said: “Absolutely there is a possibility that we as a nation could be at war with Russia, because if Russia puts one bootstep across Nato territory, we are all at war with Russia. Every single one, every single member of the Nato alliance.

“Article 5 (of the Nato alliance) says an attack on one is an attack on all, so we need to change our mindset fundamentally, and that is why I say our defence starts in the UK on the frontiers of Nato.”

The warning comes after Russia launched a military invasion of Ukraine overnight.

Mr Putin announced the action during a televised address early on Thursday morning, saying the move was a response to threats from Ukraine.

He said Russia does not have a goal to occupy Ukraine, but the responsibility for bloodshed lies with the Ukrainian “regime”.

He also warned other countries that any attempt to interfere with the Russian action would lead to “consequences they have never seen”.

Mr Putin accused the US and its allies of ignoring Russia’s demand to prevent Ukraine from joining Nato and offer Moscow security guarantees.

He said the Russian military operation aims to ensure a “demilitarisation” of Ukraine, adding that all Ukrainian servicemen who lay down arms will be able to safely leave the zone of combat.

Explosions could be heard in the Ukrainian capital of Kyiv shortly after Mr Putin’s address, while explosions were also reported in the cities of Odesa and Kharkiv.