Liz Truss will miss a trip to Ukraine after she tested positive for Covid, shortly after appearing in the Commons without wearing a mask.
The foreign secretary had been due to travel to Kyiv with Boris Johnson today in a show of support as tensions with Russia mounted.
But in a tweet on Monday evening, Truss revealed she would have to isolate and work from home.
Truss had sat on the government frontbench on Monday afternoon as Johnson faced MPs over Sue Gray’s report on the partygate saga.
She was sat next to health secretary Sajid Javid, who was wearing a mask, and home secretary Priti Patel, who was not.
Truss remained in the Commons after Johnson’s two-hour grilling to deliver a statement on Russia.
In the early evening Truss attended a meeting, to which all Conservative MPs were invited, as Johnson tried to rally support and see off a leadership challenge.
Dominic Raab, the deputy prime minister, said he was “not sure that would have made the difference” if Truss had worn a mask in the Commons chamber.
“But look, the rules were followed clearly because of the - I mean she’s been vaccinated,” he told Times Radio on Tuesday morning.
“We’re actually in a strong position to open up without being fearful.
“And of course, there’s some common sense precautions that people can decide for themselves, what and when to take.”
Nadhim Zahawi, the education secretary and former vaccines minister, also announced yesterday morning that he had tested positive and was isolating at home.