Britain and Ukraine will on Thursday sign a "strategic partnership agreement" to support Kiev's sovereignty "in the face of Russia's destabilising behaviour", Prime Minister Boris Johnson's office said.
The deal to intensify the two countries' political, trade and strategic cooperation will be signed in Downing Street as Ukrainian President Volodymyr Zelensky makes a two-day visit to the UK.
It will replace key elements of an existing partnership agreement between Ukraine and the European Union, as London looks to its post-Brexit future in 2021.
Britain formally left the bloc in January but has remained bound by most of its rules until the end of the year under the terms of its divorce.
"The UK is Ukraine's most fervent supporter," Johnson said in a statement ahead of the signing.
"We are utterly committed to upholding the sovereignty and territorial integrity of Ukraine," he added, calling the new agreement "the next chapter" in the countries' relationship.
The new deal with Kiev includes a comprehensive preferential free trade agreement covering goods, services, and tariffs and quotas, his office said.
It also details commitments to cooperate on peaceful conflict resolution, defence and security, climate change and human rights.
Russia in 2014 annexed Crimea from Ukraine, a move that also prompted the outbreak of a Moscow-backed separatist conflict in the country's east.
London said it is "a staunch defender of Ukraine's right to self-determination" and noted British troops had trained over 18,000 members of Ukraine's armed forces since 2015.
The UK will also soon lead a multi-national "maritime training initiative" for the Ukrainian Navy, according to Downing Street.