Prime Minister Rishi Sunak says the destruction of the Nova Kakhovka dam in Ukraine would mark a "new low" in the conflict if Russian forces were found to be responsible.
Ukraine accused Russian forces of blowing up the major dam and hydroelectric power station in a part of southern Ukraine that Russia controls.
Reports on social media said 11 of the 28 spans of the dam have been destroyed, allowing millions of gallons of water to flow into the Dnipro River.
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Russian military bloggers say the dam was blown at around 2am on Tuesday but, like much of the information being released, it cannot be verified, leaving many to question who, in fact, is responsible for the attack.
The Prime Minister, who spoke to reporters as he travelled to Washington for talks with US President Joe Biden, said if intentional, it would be "the largest attack on civilian infrastructure" since the beginning of the conflict.
He said that attacks on civilian infrastructure were "appalling and wrong".
Mr Sunak explained: "Our military and intelligence agencies are currently looking at it, so it's too soon to pre-empt that and make a definitive judgment.
"But what I can say is if it is intentional, it would represent, I think, the largest attack on civilian infrastructure in Ukraine since the start of the war, and just would demonstrate the new lows that we would have seen from Russian aggression.
"Attacks on civilian infrastructure are appalling and wrong. We've seen previous instances of that in this conflict so far, but it's too early to say definitively."
Mr Sunak said he would be discussing the situation with President Biden.
Foreign Secretary James Cleverly described it as an "abhorrent act", adding that "intentionally attacking exclusively civilian infrastructure is a war crime".
There are concerns that the damage to the dam could have broad consequences: flooded homes, streets and businesses downstream; depleted water levels upstream that help cool Europe's largest nuclear power station; and drained supplies of drinking water to the south in Crimea, which Russia illegally annexed.
The deputy head of the Office of the President of Ukraine said the "ecocide" was "really terrible".
Igor Zhovkva told BBC Radio 4's World At One programme Russia's claims that shelling had caused the damage were "absurd," saying: "I know that there was a blast and it was made on purpose because you cannot ruin this dam (only) by shelling."
But Evgeny Popov, a member of the Duma for Vladimir Putin's United Russia party, claimed Ukraine would "profit" from the damage and said Russia is evacuating 22,000 people from the area.
He accused Ukraine of a "war crime" by carrying out the attack.
"We don't need to do that. We are not attacking civilian infrastructure," he told the same programme.