Secretary of State Liz Truss will miss a trip to Ukraine after testing positive for the coronavirus.
Ms Truss announced on Twitter Monday night that she had tested positive and would be working from home during isolation.
But she was due on Tuesday to visit Prime Minister Boris Johnson for a meeting with Ukrainian President Volodymyr Zelenskyy amid rising tensions with Russia.
Earlier, Ms Truss announced plans for legislation with new powers to sanction individuals and companies linked to the Russian state before visiting.
She also appeared in the House of Commons – without wearing a mask – to show support for the Prime Minister during his statement on the Partygate saga.
She sat next to England’s Health Secretary Sajid Javid, who was wearing a mask, and Home Secretary Priti Patel, who was not.
Ms Truss then later attended a meeting attended by all members of the parliamentary Conservative Party, where the Prime Minister addressed MPs and colleagues to rally support after what was described as a “difficult” day.
In a tweet, Ms Truss said: “I tested positive for Covid tonight.
“Luckily I had my three shots and will be working from home while I isolate.”