Russian forces have seized Zaporizhzhia nuclear plant, the largest in Europe, according to Ukrainian authorities.
A fire earlier broke out near radioactive material at the site in the south east of the country after fierce fighting broke out, which damaged a nearby building used for training purposes and was later extinguished.
Russian troops have now seized the plant, although the local authority said in a statement personnel are "monitoring the condition of power units".
Ukraine said enemy forces attacked the plant in the early hours of Friday, with shelling attacks setting an adjacent five-story training facility on fire.
President Volodymyr Zelensky said in a video address Russian tanks had shot at the nuclear reactor plants, although there was no evidence they had been hit, and that he feared the attack was the "end of Europe".
Boris Johnson has called for an urgent meeting of the UN security council over the firefight which he said ' threatens the safety of all of Europe'.
A Downing Street spokeswoman said: “The Prime Minister spoke to Ukrainian President Zelensky in the early hours of this morning about the gravely concerning situation at Zaporizhzhia nuclear power station.
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“Both leaders agreed that Russia must immediately cease its attack on the power station and allow unfettered access for emergency services to the plant."
U.S. Energy Secretary Jennifer Granholm said on Thursday the reactors at Zaporizhzhia power station "are protected by robust containment structures and reactors are being safely shut down".
She said on Twitter she had spoken with Ukraine's energy minister about the situation at the plant and there are so far no elevated radiation readings near the facility.
Deputy Prime Minister Dominic Raab told Times Radio: "It is clearly reckless, irresponsible and not only the fact they were shooting, bombarding that particular site, but when the Ukrainian emergency authorities were trying to put out the fire, the shelling continued.
"We support the Ukrainians in dealing with the security situation there but also I think come down hard on Vladimir Putin.
"That's why the Prime Minister has called for an emergency United Nations security council meeting in New York so the entire international community can address this, because of course it is a much wider threat, given the nuclear implications.
"It is an affront to the world at large."
"It must stop.
Earlier Ukraine's regulatory authority said Russian infantry troops were moving directly towards the site of Ukraine’s largest nuclear power plant.
It said: "The battle is going on in the town of Enerhodar and on the road to the ZNPP (Zaporizhzhia Nuclear Power Plant) site."
It added the situation was “critical”, hours before the area fell to the invading army.
The move comes days after Russian forces seized Chernobyl after intense gunfire at the site of the world's worst nuclear disaster.
Fighting between Ukrainian and Russian forces in the area in the north of the country were feared to have damaged the facility.
Large parts of Chernobyl were closed to the public for decades after a nuclear power reactor melted down there during the Soviet Union's reign in 1986.
