Ukrainian forces have recaptured an area at least twice the size of Greater London, according to the UK.
In recent days Ukraine launched a swift counter-offensive that has driven Russian troops out of swaths of territory in the east.
It has forced Moscow to withdraw its troops to prevent them from being surrounded and leave behind significant numbers of weapons and munitions in a hasty retreat.
In retaliation, Russia attacked power stations and other infrastructure across Ukraine, causing widespread outages.
In its latest defence intelligence update on the situation in Ukraine on Monday, the British Ministry of Defence (MoD) said: “In the face of Ukrainian advances, Russia has likely ordered the withdrawal of its troops from the entirety of occupied Kharkiv Oblast west of the Oskil River.
“Isolated pockets of resistance remain in this sector, but since Wednesday, Ukraine has recaptured territory at least twice the size of Greater London.
The MoD added: “In the south, near Kherson, Russia is likely struggling to bring sufficient reserves forward across the Dnipro River to the front line.
“An improvised floating bridge Russia started over two weeks ago remains incomplete; Ukrainian long-range artillery is now probably hitting crossings of the Dnipro so frequently that Russia cannot carry out repairs to damaged road bridges.
“The rapid Ukrainian successes have significant implications for Russia’s overall operational design.
“The majority of the force in Ukraine is highly likely being forced to prioritise emergency defensive actions.”
“The already limited trust deployed troops have in Russia’s senior military leadership is likely to deteriorate further.”
The Russians’ pullback marked the biggest battlefield success for Ukrainian forces since they thwarted a Russian attempt to seize the capital, Kyiv, at the start of the nearly seven-month war.
Ukraine’s attack in the Kharkiv region came as a surprise for Moscow, which had relocated many of its troops from the area to the south in expectation of the main Ukrainian counter-offensive there.