The U.K.-Ukraine Political, Free Trade, and Strategic Partnership Agreement was signed Thursday, the second day of Ukrainian President Volodymyr Zelenskiy's visit to Britain.

The deal "lays the foundation for an intensified bilateral relationship in areas including trade, defense and political cooperation," Johnson's office said in statement.

It replaces key elements of an existing partnership agreement between Ukraine and the European Union.

Britain formally left the bloc in January but remains bound by most of its rules until the end of the year. It is now looking to its post-Brexit future in 2021.

Russia illegally annexed Crimea from Ukraine in 2014 and threw its support behind separatists in Ukraine's east, where some 13,200 people have been killed in an ongoing conflict.

During their meeting in Downing Street, Johnson and Zelenskiy discussed "the importance of working together to counter Russia's malign influence, both in Ukraine and in the wider region," according to the prime minister's office.

The statement said Johnson "stressed that reform and rule of law drive economic growth, and reiterated the UK's commitment to Ukraine's reform programme."

In a separate statement, Johnson said his country was Ukraine's "most fervent supporter."

"Whether it's our defense support, stabilization efforts, humanitarian assistance or close cooperation on political issues, our message is clear: we are utterly committed to upholding the sovereignty and territorial integrity of Ukraine," he said.

Britain has been "a staunch defender of Ukraine's right to self-determination," the statement noted, adding that British troops had trained more than 18,000 members of Ukraine's armed forces since 2015.

The new deal, which Johnson said "signals the next chapter in our relationship," includes a comprehensive preferential free trade agreement covering goods and services, tariffs and quotas, his office said.

It also details commitments to cooperate on peaceful conflict resolution, defense and security, climate change and human rights.

Ukrainian Defense Minister Andriy Taran and the British Secretary of State for Defense Ben Wallace on Oct. 7 signed a memorandum on strengthening cooperation in the defense sector.