The Russian army was left in “disarray and disorganisation” by Ukraine’s surprise invasion, according to UK intelligence.
Kyiv launched a surprise incursion last week in a move described as a “major provocation” by Vladimir Putin.
British Challenger 2 tanks were used by Ukraine’s forces during the military operation.
In their latest intelligence update on the war, the Ministry of Defence said Ukrainian troops had managed to advance up to 25 kilometres inside Russia’s Kursk region.
The MoD said: “Although Russia had established defensive lines and had border forces in the area, these were not prepared to respond to a multi-battalion assault force.
“After initial disarray and disorganisation, Russian forces have deployed in greater force to the region, including likely from elsewhere along the contact line.
“They have also begun to construct additional defensive positions in an effort to prevent Ukrainian advances.”
The Ukraine army’s 82nd Air Assault Brigade, which has been using British tanks, took part in the incursion.
Reports from Moscow say more than 8,000 Russians have been evacuated, and more than 6,000 put into temporary accommodation centres.
Earlier this week, Vladimir Putin said Ukraine was waging war against Russia “with the help of its western masters”.
“The enemy will continue to try to destabilise the situation in the border zone, in order to shake up the internal political situation in our country,” the president said, according to Reuters.
Ukraine’s president Volodymyr Zelenkyy said: “It is only fair to destroy Russian terrorists where they are, where they launch their strikes from. Russian military airfields, Russian logistics.
“We see how useful this can be for bringing peace closer. Russia must be forced into peace if Putin wants to continue waging war so badly.
“Russia brought war to others, and now it is coming home.”
