The UK's MOD said the number of fleeing Russians likely exceeds Putin's original invasion force.
Reports say least 190,000 Russians have 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 MOD said.
The number of Russians leaving the country to avoid President Vladimir Putin's latest mobilization in Ukraine is likely bigger than the original invasion force, according to UK intelligence.
The UK's Ministry of Defence tweeted this estimate on Thursday, writing that there has been a "considerable exodus of Russians seeking to evade call-up.
"Whilst exact numbers are unclear, it likely exceeds the size of the total invasion force Russia fielded in February 2022."
Since Putin announced on September 21 that he would call up 300,000 reservists, there has been a mass rush for the border as Russians attempt to evade being sent to the Ukrainian frontlines.
Putin had pledged in March not to introduce conscription, a move that has helped to keep Russian civilians at a remove from the realities of the Ukraine invasion. That was sharply interrupted with last week's announcement.
According to the Associated Press, as of Wednesday at least 194,000 Russians have fled to Georgia, Kazakhstan and Finland alone.
Bloomberg, citing data from the EU, Georgia, and Kazakhstan, put the figure at 200,000.
Those conservative estimates already exceed what the US had estimated to be the 190,000-strong invasion force that massed at Ukraine's border just prior to the invasion.
On Monday, satellite imaging company Maxar shared pictures of huge traffic build-ups at the border with Georgia and Mongolia.
Ten thousand people have crossed into Georgia per day since the announcement, which is double the normal traffic, according to The Washington Post.
Plane tickets to countries with friendly visa agreements with Russia also sold out almost immediately after Putin's announcement, with reports of seats on private jets going for up to $27,000, as The Guardian reported.
The UK's MOD noted on Thursday that "the better off and well educated are over-represented amongst those attempting to leave Russia."
Russia's ultra-rich were on the move long before this announcement. According to wealth intelligence firm New World Wealth, by mid-March 15,000 Russian millionaires were projected to leave the country.
The situation is likely to compound the country's economic woes and an ongoing "brain drain" effect, the ministry said.