Admiral Sir Tony Radakin compared the impact of Russia invading Ukraine to World War II.
He said the significance of a full invasion "would be on a scale not seen in Europe since World War II."
Ukraine has warned that Russia was amassing troops on its border, and that it may invade soon.
The head of the UK's armed forces warned that a full invasion of Ukraine by Russia would be the most significant in Europe since World War II.
Admiral Sir Tony Radakin in a speech on Tuesday, according to Sky News: "The significance of the worst scenarios in terms of a full invasion of Ukraine would be on a scale not seen in Europe since World War II."
Fears are growing that Russia could be preparing to invade Ukraine, potentially as soon as the end of this year or the start of 2022.
Both Ukraine and the US have warned of a potential invasion, and experts told Insider's Ryan Pickrell last month that a Russian invasion of Ukraine was a "very real possibility."
The head of Ukraine's defense intelligence agency told Military Times last month that Russia already has around 100,000 troops on its borders.
The US has been warning allies that Russia could decide to invade Ukraine in the next few months, and President Joe Biden has spoke to the leaders of France, Germany, the UK, and Italy earlier this week about how to stop such a move.
Russia has repeatedly denied any plans to invade Ukraine.
Russia annexed Ukraine's Crimea peninsula in 2014.
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