The UK said the number of fleeing Russians likely exceeded Putin's original invasion force.
Reports said least 190,000 Russians had left since Putin said he would draft reservists for Ukraine.
The exodus is likely to affect Russia's economy and add to "brain drain," the UK said.
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The number of Russians leaving the country to avoid President Vladimir Putin's latest mobilization for the war in Ukraine is likely bigger than the original invasion force, UK intelligence said.
The UK's Ministry of Defense tweeted this estimate on Thursday, writing that there had been a "considerable exodus of Russians seeking to evade call-up.".
According to the wealth-intelligence firm New World Wealth, by mid-March, 15,000 Russian millionaires were projected to leave the country. »