LONDON: Britain and Ukraine on Thursday struck a deal to support Kiev's sovereignty and counter "Russia's malign influence", Prime Minister Boris Johnson's office said.

The deal to intensify the two countries' political, trade and strategic cooperation was signed in Downing Street during a two-day visit to the UK by Ukrainian President Volodymyr Zelensky.

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 on Twitter following his meeting with Zelensky.