Boris Johnson appeared to shake hands with Brazil's health minister before the official tested positive for Covid during a trip to New York City.
Marcelo Queiroga tested positive for the disease hours after accompanying President Jair Bolsonaro to the United Nations General Assembly on Tuesday.
He will remain in New York in quarantine, the government's communications office said.
"The minister is doing well," a statement added, saying that the rest of the delegation had tested negative for the virus.
Brazilian journalist Raquel Krähenbühl released footage on Twitter that appeared to show the British Prime Minster shaking hands with Mr Queiroga before the diagnosis.
She captioned the footage: "Brazilian Health Minister - who tested positive for Covid - was staying at the same hotel as President Biden in NYC.
"He went to the UN today to watch President Bolsonaro’s speech.
"Here, he shakes hands with Prime Minister Boris Jonhson- who met Biden today at the White House."
Mr Queiroga told CNN Brasil that he had worn a mask the entire time that he was in the UN building.
His Covid test comes as Mr Johnson met US President Joe Biden at the White House.
The President and PM were looking to build bridges after the fall of Kabul during their historic meeting at the White House on Tuesday.
Mr Johnson had insisted that Britain was "first in line" for an agreement after Donald Trump took office.
But asked whether that was still the case, Mr Biden told reporters tonight: "We're going to talk about trade a little bit today and we're gonna have to work that through.”
Meanwhile, Brazil's president Jair Bolsonaro, a vaccine sceptic, defied UN rules that asked all those attending the assembly to be vaccinated and has bragged about not getting vaccinated.
In his speech to world leaders at a meeting where the global coronavirus pandemic was top of the agenda, Bolsonaro said his government did not support the use of a vaccine passport and stood by treatment of the virus with unproven drugs.
Image:AFP via Getty Images)
He said Brazil's vaccination campaign against Covid had been successful and that all adults who wanted a vaccine would be fully inoculated by November.
With unvaccinated diners barred from eating in New York restaurants, Bolsonaro and his entourage ate pizza on a sidewalk on their first night in the city on Sunday.
The health minister was the second member of the Brazilian delegation in New York to have tested positive.
A diplomat who was part of an advance team preparing Bolsonaro's visit tested positive on the weekend and was isolated.
On a trip to the US to meet former President Donald Trump at his Mar-a-Lago estate last year, 22 members of Bolsonaro's delegation caught coronavirus in the worst case of transmission in a government that ignored precautions.
Bolsonaro, who regarded Trump as a political role model, has opposed lockdowns to prevent the spread of Covid and seldom uses a maskRead More Read More