Britain said on Monday that if Russia takes action against Ukraine, it will impose sanctions on companies and individuals with the closest ties to the Kremlin, provoking a warning from Moscow that it will respond if its businesses are "attacked."

Since the demise of the Soviet Union in 1991, London has emerged as the preeminent global hub for a massive outflow of funds from former Soviet republics.

Opponents of President Vladimir Putin, who has massed forces in Ukraine, have repeatedly urged the West to crack down on Russian money, despite the fact that oligarchs and Russian officials continue to flaunt their wealth in Europe's most luxurious destinations.

"We are absolutely clear that if Russia takes more action against Ukraine, we will tighten the sanctions regime targeting those businesses and individuals with the closest ties to the Kremlin," said Simon Clarke, Chief Secretary to the Treasury.

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Reports that the UK is considering further sanctions against Russian oligarchs' financial holdings in the country are "very alarming," according to Kremlin spokesman Dmitry Peskov.

Such restrictions, Peskov said, would be "a blatant attack on business," noting that many Russian businesses had UK partners and shareholders who would be harmed by such restrictions.

“Any attack against Russian firms would necessitate a response, and such actions, if necessary, would be based on Russian interests,” he added.

The US, the EU, and the UK have all warned Putin not to attack Ukraine. Russia denies any plans to attack Ukraine and is demanding security guarantees, including a vow from NATO that Kyiv will never be admitted to the alliance.