Foreign Secretary Liz Truss has warned Russia it will face “severe economic consequences” if it invades Ukraine.
She said G7 foreign ministers meeting in Liverpool this weekend would put on a show of unity and make clear such a move would be a “strategic mistake”.
Ms Truss said the UK and its allies had to “deter Russia from taking that course of action”.
Tensions are growing as Moscow amasses troops on Ukraine’s border – but the Kremlin has denied it plans to invade.
Reiterating previous warnings from the US and its allies, Ms Truss said the G7 “are going to absolutely be strong in our stance against aggression… with respect to Ukraine”.
She added: “If Russia were to take that action, it would be a strategic mistake, and there will be severe consequences for Russia.
“And what we’re doing this weekend is working with like-minded allies to spell that out.”
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Ukraine shares borders with both the EU and Russia, but as a former Soviet republic it has deep social and cultural ties with Russia.
However, Russia has accused Ukraine of provocation, and sought guarantees against eastward Nato expansion and deployment of weapons close to its border.
Ahead of the G7 meeting, Ms Truss held talks with Ukrainian foreign minister Dmytro Kuleba in London.
Asked if she could rule out a military response to Russian aggression, Ms Truss said the UK was working with Ukraine on “defence and security capability”.
“We are providing them with support,” she said. “We are also working to help them with their energy resilience. So they’re not solely dependent on Russian energy supplies. And that is the way ultimately that we will help support Ukraine.”
Ms Truss said the UK and EU were trying to ensure Western economies were less dependent on Russian energy.
The foreign secretary said the Nord Stream 2 gas pipeline between Germany and Russia would be “a problem” if Russia invaded Ukraine.
It would be an issue to discuss with her new German counterpart at the G7 talks.
Ms Truss said: “We absolutely need to reduce dependence on Russian gas and energy. And this should be part of an overall strategy of reducing dependence on non market economies, reducing dependence on malign actors, and making sure that the free world is able to have that strategic independence it needs to survive and thrive.”
US President Joe Biden and Russian President Vladimir Putin on Tuesday held a virtual summit aimed at reducing tensions in the region. Mr Biden said he had made it clear to Mr Putin that there would be “economic consequences like none he’s ever seen” if he invaded Ukraine.
Additional reporting by Alex Kleiderman