"We are behind you and that will not change," Andy Burnham has told Volodymyr Zelensky, as he hosted the Ukrainian leader in Downing Street.
Zelensky was in London as part of efforts to reinforce backing from allies after Donald Trump called for him to go and lifted sanctions on Russian diesel.
The prime minister said Ukraine was willing to discuss an energy ceasefire proposed by the US president but said Russia's response had been "further attacks on critical national infrastructure in Ukraine".
Zelensky said: "Our people, we survived and continue to do this because we have strong partners and I'm very proud that we have one of the strongest partners in our history and it's your people, Andy."
The Ukrainian president was greeted with a hug from Burnham, cheers from civil servants and a rare honour guard as he arrived in Downing Street.
Ahead of Zelensky's arrival, No 10 announced it would speed up delivery of lightweight multirole missiles to Ukraine, which have been used to bring down Russian drones.
Ukrainian officials have said that overnight strikes by Russian missiles and drones had left at least 600,000 people without electricity.
For the second consecutive night, Ukraine said it had not received advanced warning from its allies of a wave of missile strikes coming from Russia.
Zelensky has suggested the US had threatened to restrict intelligence-sharing unless he agreed to sign up to an energy ceasefire. Trump has denied the US is holding back information.
Speaking in Downing Street, Burnham said the recent days had been "dark and difficult" for Ukraine, with the country suffering "heavy bombardment".
He told the Ukrainian leader: "You have seen the solidarity and the support you have got from everybody here in Britain.
"We are behind you and that will not change."
Earlier in the day, Burnham had been invited to criticise the US decision to lift sanctions on Russia, when, at Prime Minister's Questions, Lib Dem leader Sir Ed Davey asked if the PM agreed there had been a "shameful Trump betrayal".
Burnham acknowledged that Zelensky was "carrying the heaviest of weights" but avoided attacking Trump, instead telling MPs that his role was to "repair bridges so that the support comes back".
"Therefore I should, at this moment in time, be careful in terms of laying the ground to bring people back together."
He added that Zelensky would not come away from his trip "empty-handed".
On Friday, Trump shocked European allies when he announced that he had reached a deal with Russian President Vladimir Putin to release an initial 300,000 tonnes of diesel, with more to follow.
Concerns about the cost of living are playing a key part in the US's November elections and the US president hopes his deal with Russia will reduce the cost of diesel.
The agreement runs counter to sanctions imposed on Russia by the US and its allies in a bid to choke off funds for Putin's war in Ukraine.
The deal came days after Trump accused Zelensky of wanting to "make problems for the world" and that it was time for Ukraine to "get a new president".
The criticism marks a renewal of Trump's previous hostility towards Zelensky which resulted in an infamous public row between the two men in the Oval Office.