His continued presence in the House of Commons on the backbenches will be closely watched, so will his interventions when the next prime minister takes over
Ukrainian defenders battled on Saturday to contain Russian forces along several fronts, officials said.
A missile strike on the northeastern city of Kharkiv wounded three civilians, its governor said, though Russia's main attacks appeared focused southeast of there in Luhansk and Donetsk.
Those two provinces, parts of which were held by pro-Russian separatists before the conflict began in February, comprise the eastern industrial region of the Donbas.
Ukrainian officials reported strikes in both on Saturday, while Britain's Ministry of Defence said Moscow was assembling reserve forces from across Russia near Ukraine.
Donetsk regional Governor Pavlo Kyrylenko said on the Telegram messaging service that a Russian missile had struck Druzhkivka, a town behind the front line, and reported shelling of other population centres.
Luhansk Governor Serhiy Gaidai said on Telegram that Russian forces were "firing along the entire front line", though a subsequent Ukrainian counter-attack had forced Moscow to halt its offensive.
Ukraine President Volodymyr Zelensky said the Russian army had targeted civilians on purpose.
"It fired precisely at the residential sector – absolutely deliberately, purposefully, at ordinary houses and civilian objects," he said in a statement.
Russia, which claimed control over all of Luhansk province last weekend, denies targeting civilians.
Zelensky also dismissed several of Kyiv's senior envoys abroad, saying it was part of "normal diplomatic practice." He said he would appoint new ambassadors to Germany, India, the Czech Republic, Norway and Hungary.
Zelensky has urged his diplomats to drum up international support and high-end weapons to slow Russia's advance.
US President Joe Biden signed a weapons package for Ukraine on Friday worth up to $400 million, including four additional high mobility artillery rocket systems (HIMARS).
But Ukraine suffered a diplomatic setback on Saturday, when Canada said it would return a repaired turbine that Russia's state-controlled Gazprom used to supply natural gas to Germany. Ukraine had argued that a return would violate sanctions on Russia.
Following Friday's testy G20 exchanges, President Vladimir Putin also signalled that the Kremlin was in no mood for compromise, saying sanctions against Russia risked causing "catastrophic" energy price rises.
Ukraine's Foreign Minister Dmytro Kuleba said on Saturday that sanctions were working, and echoed calls for more deliveries of high-precision Western weapons.
"Russians desperately try to lift those sanctions which proves that they do hurt them. Therefore, sanctions must be stepped up until Putin drops his aggressive plans," Kuleba told a forum in Dubrovnik by videolink.
Since Russia, which has also seized a big chunk of territory across Ukraine's south, started what it calls a "special operation" to demilitarize Ukraine, cities have been bombed to rubble, thousands have been killed, and millions displaced.
Ukraine and its Western allies say Russia is engaged in an unprovoked land grab.
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