“Putin cannot be allowed to alter international borders using brute force,” Truss said in a statement.
"We will ensure he loses this illegal war," she added.
She also stated that Russian President Vladimir Putin "has, once again, acted in violation of international law with clear disregard for the lives of the Ukrainian people he claims to represent.”
Later on Friday, Putin was expected to announce the incorporation into Russia of four Ukrainian regions where Russian separatists held what they called referendums from September 23 to 27.
The four regions combined account for around 15 percent of Ukraine, making Putin's move the biggest annexation in Europe since Hitler, according to the Reuters news agency.
Friday is day 219 of Russia's war on Ukraine.
Source: PAP, Reuters