The Queen posed in front of blue-and-yellow flowers in a gesture of support for Ukraine at her first in-person meeting since falling ill with Covid.
A Royal source said the Ukrainian flag’s colours were chosen as a sign of solidarity as she held her audience with the Canadian prime minister.
The 95-year-old monarch was pictured smiling warmly at Justin Trudeau as he held her right hand in both of his.
Mr Trudeau is in the UK for talks with Boris Johnson about the Ukraine war.
The monarch, who is also Queen of Canada, tested positive for Covid-19 on 20 February and has carried out only “light duties” since then, cancelling several events.
She carried out her first virtual engagements on 1 March before Monday’s face-to-face meeting.
During the audience at Windsor Castle’s Oak Room, Mr Trudeau was also pictured laughing as the Queen gestured towards herself.
The Queen was not seen with the walking stick which she has been using recently.
As well as this gesture of support for Ukraine at the meeting, the Queen previously made a donation to the Disasters Emergency Committee appeal to help people fleeing the conflict in Ukraine.
She has two high-profile events scheduled in the coming weeks: the Commonwealth Service on 14 March and the Duke of Edinburgh’s memorial service on 29 March, both at Westminster Abbey.
Last month, she became the nation’s longest reigning monarch, reaching her Platinum Jubilee after 70 years on the throne.