Mykhailo Podolyak said in a Twitter post that Johnson, who on Thursday announced his resignation, was a true leader who was able to "take responsibility in the hardest of times" and who was "the first to arrive in Kyiv, despite missile attacks" during Russia's invasion.
Johnson said on Thursday he would step down as Britain's prime minister, bowing to calls from ministerial colleagues and lawmakers in his Conservative Party, the Reuters news agency reported.
He added he would remain in office until a new prime minister is elected.
'Leading the West in standing up to Putin's aggression in Ukraine'
In a resignation speech outside Downing Street, Johnson said he was "immensely proud" of the achievements of his government, "from getting Brexit done to settling our relations with the continent for over half a century, reclaiming the power for this country to make its own laws in parliament, getting us all through the pandemic, delivering the fastest vaccine rollout in Europe, the fastest exit from lockdown, and in the last few months, leading the West in standing up to Putin's aggression in Ukraine."
He added: "And let me say now, to the people of Ukraine, that I know that we in the UK will continue to back your fight for freedom for as long as it takes."
Johnson in early May became the first Western leader to deliver a remote address to the parliament in Kyiv, where he was given numerous standing ovations for his steadfast support of the Ukrainian cause.
In late April, he promised to step up military aid to Ukraine amid Russia's offensive in the eastern Donbas region.
Last week he announced that Britain would provide another 1 billion pounds (USD 1.2 billion, EUR 1.16 billion) of military support to Ukraine.
(gs)
Source: PAP, Reuters, CNN